ISO 24610-1:2006

INTERNATIONAL ISO
STANDARD 24610-1
FIrst edition
2006-04-15

Language resource management —
Feature structures —
Part 1:
Feature structure representation
Gestion des ressources linguistiques — Structures de traits —
Partie 1: Représentation de structures de traits




Reference number
ISO 24610-1:2006(E)
©
ISO 2006

---------------------- Page: 1 ----------------------
ISO 24610-1:2006(E)
PDF disclaimer
This PDF file may contain embedded typefaces. In accordance with Adobe's licensing policy, this file may be printed or viewed but
shall not be edited unless the typefaces which are embedded are licensed to and installed on the computer performing the editing. In
downloading this file, parties accept therein the responsibility of not infringing Adobe's licensing policy. The ISO Central Secretariat
accepts no liability in this area.
Adobe is a trademark of Adobe Systems Incorporated.
Details of the software products used to create this PDF file can be found in the General Info relative to the file; the PDF-creation
parameters were optimized for printing. Every care has been taken to ensure that the file is suitable for use by ISO member bodies. In
the unlikely event that a problem relating to it is found, please inform the Central Secretariat at the address given below.


©  ISO 2006
All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized in any form or by any means,
electronic or mechanical, including photocopying and microfilm, without permission in writing from either ISO at the address below or
ISO's member body in the country of the requester.
ISO copyright office
Case postale 56 • CH-1211 Geneva 20
Tel. + 41 22 749 01 11
Fax + 41 22 749 09 47
E-mail copyright@iso.org
Web www.iso.org
Published in Switzerland

ii © ISO 2006 – All rights reserved

---------------------- Page: 2 ----------------------
ISO 24610-1:2006(E)
Contents Page
Foreword. v
Introduction . vi
1 Scope. 1
2 Normative references. 1
3 Terms and definitions. 1
4 General characteristics of feature structure. 4
4.1 Overview. 4
4.2 Use of feature structures . 4
4.3 Basic concepts. 5
4.4 Notations . 5
4.4.1 Overview. 5
4.4.2 Graph notation . 6
4.4.3 Matrix notation . 7
4.4.4 XML-based notation. 8
4.5 Structure sharing. 10
4.6 Collections as complex feature values. 12
4.6.1 Overview. 12
4.6.2 Lists as feature values . 12
4.6.3 Sets as feature values . 14
4.6.4 Multisets as feature values . 15
4.7 Typed feature structure. 16
4.7.1 Overview. 16
4.7.2 Types. 16
4.7.3 Notations . 16
4.8 Subsumption: relation on feature structures . 18
4.8.1 Overview. 18
4.8.2 Definition . 18
4.8.3 Condition A on path values . 19
4.8.4 Condition B on structure sharing . 19
4.8.5 Condition C on type ordering . 20
4.9 Operations on feature structures and feature values. 21
4.9.1 Overview. 21
4.9.2 Compatibility . 21
4.9.3 Unification . 22
4.9.4 Unification of shared structures . 22
4.10 Operations on feature values and types . 23
4.10.1 Concatenation and union operations . 23
4.10.2 Alternation. 24
4.10.3 Negation. 25
4.11 Informal semantics of feature structures. 27
5 XML Representation of feature structures. 29
5.1 Overview. 29
5.2 Organization . 29
5.3 Elementary feature structures and the binary feature value. 30
5.4 Other atomic feature values . 32
5.5 Feature and feature-value libraries . 35
5.6 Feature structures as complex feature values .37
5.7 Re-entrant feature structures . 40
5.8 Collections as complex feature values. 41
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ISO 24610-1:2006(E)
5.9 Feature value expressions. 44
5.9.1 Overview. 44
5.9.2 Alternation. 44
5.9.3 Negation. 47
5.9.4 Collection of values . 48
5.10 Default values. 48
5.11 Linking text and analysis . 50
Annex A (informative) Formal definitions and implementation of the XML representation of
feature structures. 54
A.1 Overview. 54
A.2 RELAX NG specification for the module . 54
Annex B (informative) Examples for illustration . 60
Annex C (informative) Type inheritance hierarchies. 62
C.1 Overview. 62
C.2 Definition. 62
C.3 Multiple inheritance . 64
C.4 Type constraints . 64
Annex D (informative) Denotational semantics of feature structure. 66
D.1 Feature structure signatures . 66
D.2 Feature structure algebra. 66
D.3 FS domains. 67
D.4 Feature structure interpretations . 68
D.5 Satisfiability . 68
D.6 Subsumption . 68
D.7 Unification. 69
Annex E (informative) Use of feature structures in applications. 70
E.1 Overview. 70
E.2 Phonological representation. 70
E.3 Grammar formalisms or theories . 70
E.4 Computational implementations . 71
Bibliography . 75

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ISO 24610-1:2006(E)
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards bodies
(ISO member bodies). The work of preparing International Standards is normally carried out through ISO
technical committees. Each member body interested in a subject for which a technical committee has been
established has the right to be represented on that committee. International organizations, governmental and
non-governmental, in liaison with ISO, also take part in the work. ISO collaborates closely with the
International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization.
International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2.
The main task of technical committees is to prepare International Standards. Draft International Standards
adopted by the technical committees are circulated to the member bodies for voting. Publication as an
International Standard requires approval by at least 75 % of the member bodies casting a vote.
Attention is drawn to the possibility that some of the elements of this document may be the subject of patent
rights. ISO shall not be held responsible for identifying any or all such patent rights.
ISO 24610-1 was prepared by Technical Committee ISO/TC 37, Terminology and other language and content
resources, Subcommittee SC 4, Language resource management.
ISO 24610 consists of the following parts, under the general title Language resource management — Feature
structures:
⎯ Part 1: Feature structure representation
The following part is under preparation:
⎯ Part 2: Feature system declaration

© ISO 2006 – All rights reserved v

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ISO 24610-1:2006(E)
Introduction
This part of ISO 24610 results from the agreement between the Text Encoding Initiative Consortium (TEI) and
the ISO TC 37/SC 4 that a joint activity should take place to revise the two existing chapters on feature
structures and feature system declaration in The TEI Guidelines called P4.
It is foreseen that ISO 24610 will have the following two parts.
⎯ Part 1, Feature structure representation, describes feature structures and their representation. It provides
an informal but explicit overview of their basic characteristics and formal semantics. In addition, part 1
defines a standard XML (eXtended Markup Language) vocabulary for the representation of untyped
feature structures, feature values, and feature libraries. It thus provides a reference format for the
exchange of feature structure representations between different application systems.
⎯ Part 2, Feature system declaration, discusses ways of validating typed feature structures which are
conformant to part 1, and of enforcing application-specific constraints. It proposes an XML vocabulary for
the representation of such constraints with reference to a set of features and the range of values
appropriate for them, and thus facilitates representation and validation of a type hierarchy as well as other
well-formedness conditions for particular applications, in particular those related to the goal of language
resource management.

vi © ISO 2006 – All rights reserved

---------------------- Page: 6 ----------------------
INTERNATIONAL STANDARD ISO 24610-1:2006(E)

Language resource management — Feature structures —
Part 1:
Feature structure representation
1 Scope
Feature structures are an essential part of many linguistic formalisms as well as an underlying mechanism for
representing the information consumed or produced by and for language engineering applications. This part of
ISO 24610 provides a format for the representation, storage and exchange of feature structures in natural
language applications concerned with the annotation, production or analysis of linguistic data. It also defines a
computer format for the description of constraints that bear on a set of features, feature values, feature
specifications and operations on feature structures, thus offering a means of checking the conformance of
each feature structure with regards to a reference specification.
2 Normative references
The following referenced documents are indispensable for the application of this document. For dated
references, only the edition cited applies. For undated references, the latest edition of the referenced
document (including any amendments) applies.
ISO 8879, Information processing — Text and office systems — Standard Generalized Markup Language
(SGML), as extended by TC 2 (ISO/IEC JTC 1/SC 34 N029: 1998-12-06).
ISO 19757-2, Information technology — Document Schema Definition Language (DSDL) — Part 2: Regular-
grammar-based validation — RELAX NG
NOTE The first reference permits the use of XML and the second, RELAX NG,provides a specification for XML
modules. RELAX NG is a schema language for XML, standing for REgular LAnguage for XML for Next Generation, and
simplifies and extends the features of DTDs, Document Type Definitions.
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 8879 and ISO 19757-2 and the
following apply. This list is provided to clarify the terminology relating to feature structures used throughout
this part of ISO 24610. Terminology derived from XLM and other formal languages is not defined here.
3.1
alternation
operation on feature values (3.23) that returns one and only one of the values supplied as its argument
NOTE Given a feature specification F : a|b, where a|b denotes the alternation of a and b, F has either the value a or
the value b, but not both.
3.2
atomic value
value (3.23) without internal structure, i.e. value other than feature structure (3.10) and collection (3.4)
© ISO 2006 – All rights reserved 1

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ISO 24610-1:2006(E)
3.3
boxed label
label in box used in a matrix notation to denote a value shared by several features (3.8)
NOTE The label may be any alphanumeric symbol.
3.4
collection
list, set, or multiset of values (3.23)
NOTE A list is an ordered collection of entities some of which may be identical. A set is an unordered collection of
unique entities. A multiset is an unordered collection of entities that may or may not be unique; it is sometimes referred to
as a bag.
3.5
complex value
value (3.23) represented either as a feature structure (3.10) or as collection (3.4)
3.6
concatenation
operation of combining two lists of values (3.23) into a single list
3.7
empty feature structure
feature structure (3.10) containing no feature specifications (3.9)
3.8
feature
property of an entity
NOTE The combination of feature and feature-value constitutes a feature specification (3.9). For example, number
is a feature, singular is a value, and a pair is a feature specification.
3.9
feature specification
assignment of a value (3.23) to a feature (3.8)
NOTE Formally, it is treated as a pair of a feature and its value.
3.10
feature structure
set of feature specifications (3.9)
NOTE The minimum feature structure is the empty feature structure (3.7).
3.11
graph notation
notation of feature structure (3.10) in a single rooted graph
3.12
incompatibility
relation between two feature structures (3.10) which have conflicting types (3.19) or at least one common
feature (3.8) with incompatible values (3.23)
NOTE Two feature structures that are incompatible cannot be unified. The empty feature structure (3.7) is
compatible with any other feature structure.
2 © ISO 2006 – All rights reserved

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ISO 24610-1:2006(E)
3.13
matrix notation
attribute-value matrix
AVM
notation that uses square brackets to represent feature structures (3.10)
NOTE In a matrix notation, each row represents a feature specification (3.9), with the feature name and the feature
value separated by a colon (:), space ( ) or the equals sign (=).
3.14
merge
generic operation that includes union (3.22) of sets or multisets and concatenation (3.6) of lists
3.15
negation
(unary) operation on a value (3.23) denoting any other value incompatible with it
NOTE In this part of ISO 24610, negation applies to values only and is not understood as a truth function as in
ordinary bivalent logics.
3.16
path
sequence of labeled arcs connecting nodes in a graph
3.17
structure sharing
re-entrancy
relation between two or more features (3.8) within a feature structure (3.10) that share a value (3.23)
3.18
subsumption
relationship between two feature structures (3.10) in which one is more specific than the other
NOTE A feature structure A is said to subsume a feature structure B if A is at least as informative as B. Subsumption
is a reflexive, antisymmetric, and transitive relation between two feature structures.
3.19
type
name of a class of entities
NOTE Feature structures (3.10) may be characterized by grouping them into certain classes. Types are used to
name such classes.
3.20
typed feature structure
feature structure (3.10) labelled by a type (3.19)
NOTE In the graph notation (3.11), each node is labelled with a type. In the matrix notation (3.13), a type is
ordinarily placed at the upper left corner of the inside of the pair of square brackets that represents a typed feature
structure. In XML notation, the type is supplied as the value (3.23) of a type attribute on the element.
3.21
unification
operation that combines two compatible feature structures (3.10) into the least informative feature structure
that contains the information from the two
3.22
union
operation that combines two sets, or multisets, into one
NOTE The corresponding operation for lists is concatenation (3.6).
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ISO 24610-1:2006(E)
3.23
value
information about an entity
NOTE There are two kinds of feature values: atomic value (3.2) and complex value (3.5).
4 General characteristics of feature structure
4.1 Overview
A feature structure is a general purpose data structure that identifies and groups together individual features
by assigning a particular value to each. Because of the generality of feature structures, they can be used to
represent many different kinds of information. Interrelations among various pieces of information and their
instantiation in markup provide a meta-language for representing analysis and interpretation of linguistic
content. Moreover, this instantiation allows a specification of a set of features with values of specific types and
1)
restrictions, by means of feature system declarations, or other XML mechanisms discussed in ISO 24610-2 .
4.2 Use of feature structures
Feature structures provide partial information about an object by specifying values for some or all of its
features. For example, if a female employee named Sandy Jones who is 30 years old is of the present
concern, then that person’s sex, name and age can be specified in a succinct manner by assigning a value to
each of these three features. These pieces of information can be put into a simple set notation, as in:
(1) Employee
{, , }
Feature structures are generally used as a vehicle for linguistic descriptions. For example, the phoneme /p/ in
English can be analysed in terms of its distinctive features: consonantal, anterior, voiceless, non-continuant or
stop sound segment, etc. Each of these features may be combined with one or other of the binary values
plus(+) and minus(-) to provide a feature specification. In a phonemic analysis such as the following, the value
of a feature specifies the presence or absence of that feature:
(2) Sound segment /p/
{, , , }
In such an analysis, the sound segment /p/ is distinguished from other phonemes in terms of the presence or
absence of specific features. For example, /p/ differs from the phoneme /b/ in VOICING, and from /k/ in
articulatory position: one is articulated at the anterior, (the lip or alveolar area of the mouth), and the other at
the nonanterior, namely the back of the oral cavity.
This feature analysis can be extended to other kinds of description. Consider a verb like “love”. Its features
include both syntactic and semantic properties: as a transitive verb, it takes an object as well as a subject as
its arguments, expressing the semantic relation of loving between two persons or animate beings. The exact
representation of these feature specifications requires a detailed elaboration of what feature structures are.
For now, these grammatical features can be roughly represented in a set format like the following:
(3) Grammatical features of the verb “love”
{, , },
where POS stands for part of speech.

1) Under preparation.
4 © ISO 2006 – All rights reserved

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ISO 24610-1:2006(E)
Since its first extensive use in generative phonology in mid-60s, the feature structure formalism has become
an essential tool not only for phonology, but more generally in support of syntax and semantics as well as
lexicon building, especially in computational work. Feature structures are used to describe and model
linguistic entities and phenomena by specifying their properties. In the next clauses, some of the formal
properties of feature structures are outlined together with means of representing them in a systematic manner.
4.3 Basic concepts
Feature structures may be viewed in a variety of ways. The most common and perhaps the most intuitive
views are the following:
⎯ a set of feature specifications that consists of pairs of features and their values;
⎯ labelled directed graphs with a single root where each arc is labelled with the name of a feature and
directed to its value.
In set-theoretic terms, a feature structure FS can be defined as a partial function from a set Feat of features to
a set FeatVal of values, where FeatVal consists of a set AtomVal of atomic values and a set FS of feature
structures.
(4) A feature structure as a set or partial function
FS ⊆ {⎥F∈Feat,v∈FeatVal}
i i i i
or
FS : Feat ⎯→FeatVal
where FeatVal = AtomVal∪ FS and where FeatVal stands for all possible values.
Values may be regarded as either atomic or complex. Atomic values are entities without internal structure,
while complex values may be feature structures or collections of values (either complex or atomic).
NOTE These two definitions are not absolutely equivalent, for in a set there may be more than one value, v, for a
feature, F.
For example, the part of speech (POS) feature can take the name of an atomic morpho-syntactic category
such as verb as its value. Conversely, the agreement (AGR) feature in English takes a complex value in the
form of a feature structure with features PERSON and NUMBER. The word “loves”, for instance, has a POS
feature with the value “verb”, while the value of its AGR feature consists of a feature structure comprising two
feature specifications: PERSON with the value “3rd”, and NUMBER with the value “singular”.
4.4 Notations
4.4.1 Overview
As a list of feature-value pairs, the overall form of a feature structure is simple. However, the internal structure
of a feature structure may be complex when a feature structure contains either
a) a feature whose value is itself a feature structure, or
b) a multi-valued feature wh
...

SLOVENSKI STANDARD
SIST ISO 24610-1:2013
01-julij-2013
Upravljanje z jezikovnimi viri - Strukture lastnosti - 1. del: Predstavitev struktur
lastnosti
Language resource management -- Feature structures -- Part 1: Feature structure
representation
Gestion des ressources linguistiques -- Structures de traits -- Partie 1: Représentation de
structures de traits
Ta slovenski standard je istoveten z: ISO 24610-1:2006
ICS:
01.140.20 Informacijske vede Information sciences
SIST ISO 24610-1:2013 en,fr,de
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

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SIST ISO 24610-1:2013

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SIST ISO 24610-1:2013

INTERNATIONAL ISO
STANDARD 24610-1
FIrst edition
2006-04-15

Language resource management —
Feature structures —
Part 1:
Feature structure representation
Gestion des ressources linguistiques — Structures de traits —
Partie 1: Représentation de structures de traits




Reference number
ISO 24610-1:2006(E)
©
ISO 2006

---------------------- Page: 3 ----------------------

SIST ISO 24610-1:2013
ISO 24610-1:2006(E)
PDF disclaimer
This PDF file may contain embedded typefaces. In accordance with Adobe's licensing policy, this file may be printed or viewed but
shall not be edited unless the typefaces which are embedded are licensed to and installed on the computer performing the editing. In
downloading this file, parties accept therein the responsibility of not infringing Adobe's licensing policy. The ISO Central Secretariat
accepts no liability in this area.
Adobe is a trademark of Adobe Systems Incorporated.
Details of the software products used to create this PDF file can be found in the General Info relative to the file; the PDF-creation
parameters were optimized for printing. Every care has been taken to ensure that the file is suitable for use by ISO member bodies. In
the unlikely event that a problem relating to it is found, please inform the Central Secretariat at the address given below.


©  ISO 2006
All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized in any form or by any means,
electronic or mechanical, including photocopying and microfilm, without permission in writing from either ISO at the address below or
ISO's member body in the country of the requester.
ISO copyright office
Case postale 56 • CH-1211 Geneva 20
Tel. + 41 22 749 01 11
Fax + 41 22 749 09 47
E-mail copyright@iso.org
Web www.iso.org
Published in Switzerland

ii © ISO 2006 – All rights reserved

---------------------- Page: 4 ----------------------

SIST ISO 24610-1:2013
ISO 24610-1:2006(E)
Contents Page
Foreword. v
Introduction . vi
1 Scope. 1
2 Normative references. 1
3 Terms and definitions. 1
4 General characteristics of feature structure. 4
4.1 Overview. 4
4.2 Use of feature structures . 4
4.3 Basic concepts. 5
4.4 Notations . 5
4.4.1 Overview. 5
4.4.2 Graph notation . 6
4.4.3 Matrix notation . 7
4.4.4 XML-based notation. 8
4.5 Structure sharing. 10
4.6 Collections as complex feature values. 12
4.6.1 Overview. 12
4.6.2 Lists as feature values . 12
4.6.3 Sets as feature values . 14
4.6.4 Multisets as feature values . 15
4.7 Typed feature structure. 16
4.7.1 Overview. 16
4.7.2 Types. 16
4.7.3 Notations . 16
4.8 Subsumption: relation on feature structures . 18
4.8.1 Overview. 18
4.8.2 Definition . 18
4.8.3 Condition A on path values . 19
4.8.4 Condition B on structure sharing . 19
4.8.5 Condition C on type ordering . 20
4.9 Operations on feature structures and feature values. 21
4.9.1 Overview. 21
4.9.2 Compatibility . 21
4.9.3 Unification . 22
4.9.4 Unification of shared structures . 22
4.10 Operations on feature values and types . 23
4.10.1 Concatenation and union operations . 23
4.10.2 Alternation. 24
4.10.3 Negation. 25
4.11 Informal semantics of feature structures. 27
5 XML Representation of feature structures. 29
5.1 Overview. 29
5.2 Organization . 29
5.3 Elementary feature structures and the binary feature value. 30
5.4 Other atomic feature values . 32
5.5 Feature and feature-value libraries . 35
5.6 Feature structures as complex feature values .37
5.7 Re-entrant feature structures . 40
5.8 Collections as complex feature values. 41
© ISO 2006 – All rights reserved iii

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SIST ISO 24610-1:2013
ISO 24610-1:2006(E)
5.9 Feature value expressions. 44
5.9.1 Overview. 44
5.9.2 Alternation. 44
5.9.3 Negation. 47
5.9.4 Collection of values . 48
5.10 Default values. 48
5.11 Linking text and analysis . 50
Annex A (informative) Formal definitions and implementation of the XML representation of
feature structures. 54
A.1 Overview. 54
A.2 RELAX NG specification for the module . 54
Annex B (informative) Examples for illustration . 60
Annex C (informative) Type inheritance hierarchies. 62
C.1 Overview. 62
C.2 Definition. 62
C.3 Multiple inheritance . 64
C.4 Type constraints . 64
Annex D (informative) Denotational semantics of feature structure. 66
D.1 Feature structure signatures . 66
D.2 Feature structure algebra. 66
D.3 FS domains. 67
D.4 Feature structure interpretations . 68
D.5 Satisfiability . 68
D.6 Subsumption . 68
D.7 Unification. 69
Annex E (informative) Use of feature structures in applications. 70
E.1 Overview. 70
E.2 Phonological representation. 70
E.3 Grammar formalisms or theories . 70
E.4 Computational implementations . 71
Bibliography . 75

iv © ISO 2006 – All rights reserved

---------------------- Page: 6 ----------------------

SIST ISO 24610-1:2013
ISO 24610-1:2006(E)
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards bodies
(ISO member bodies). The work of preparing International Standards is normally carried out through ISO
technical committees. Each member body interested in a subject for which a technical committee has been
established has the right to be represented on that committee. International organizations, governmental and
non-governmental, in liaison with ISO, also take part in the work. ISO collaborates closely with the
International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization.
International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2.
The main task of technical committees is to prepare International Standards. Draft International Standards
adopted by the technical committees are circulated to the member bodies for voting. Publication as an
International Standard requires approval by at least 75 % of the member bodies casting a vote.
Attention is drawn to the possibility that some of the elements of this document may be the subject of patent
rights. ISO shall not be held responsible for identifying any or all such patent rights.
ISO 24610-1 was prepared by Technical Committee ISO/TC 37, Terminology and other language and content
resources, Subcommittee SC 4, Language resource management.
ISO 24610 consists of the following parts, under the general title Language resource management — Feature
structures:
⎯ Part 1: Feature structure representation
The following part is under preparation:
⎯ Part 2: Feature system declaration

© ISO 2006 – All rights reserved v

---------------------- Page: 7 ----------------------

SIST ISO 24610-1:2013
ISO 24610-1:2006(E)
Introduction
This part of ISO 24610 results from the agreement between the Text Encoding Initiative Consortium (TEI) and
the ISO TC 37/SC 4 that a joint activity should take place to revise the two existing chapters on feature
structures and feature system declaration in The TEI Guidelines called P4.
It is foreseen that ISO 24610 will have the following two parts.
⎯ Part 1, Feature structure representation, describes feature structures and their representation. It provides
an informal but explicit overview of their basic characteristics and formal semantics. In addition, part 1
defines a standard XML (eXtended Markup Language) vocabulary for the representation of untyped
feature structures, feature values, and feature libraries. It thus provides a reference format for the
exchange of feature structure representations between different application systems.
⎯ Part 2, Feature system declaration, discusses ways of validating typed feature structures which are
conformant to part 1, and of enforcing application-specific constraints. It proposes an XML vocabulary for
the representation of such constraints with reference to a set of features and the range of values
appropriate for them, and thus facilitates representation and validation of a type hierarchy as well as other
well-formedness conditions for particular applications, in particular those related to the goal of language
resource management.

vi © ISO 2006 – All rights reserved

---------------------- Page: 8 ----------------------

SIST ISO 24610-1:2013
INTERNATIONAL STANDARD ISO 24610-1:2006(E)

Language resource management — Feature structures —
Part 1:
Feature structure representation
1 Scope
Feature structures are an essential part of many linguistic formalisms as well as an underlying mechanism for
representing the information consumed or produced by and for language engineering applications. This part of
ISO 24610 provides a format for the representation, storage and exchange of feature structures in natural
language applications concerned with the annotation, production or analysis of linguistic data. It also defines a
computer format for the description of constraints that bear on a set of features, feature values, feature
specifications and operations on feature structures, thus offering a means of checking the conformance of
each feature structure with regards to a reference specification.
2 Normative references
The following referenced documents are indispensable for the application of this document. For dated
references, only the edition cited applies. For undated references, the latest edition of the referenced
document (including any amendments) applies.
ISO 8879, Information processing — Text and office systems — Standard Generalized Markup Language
(SGML), as extended by TC 2 (ISO/IEC JTC 1/SC 34 N029: 1998-12-06).
ISO 19757-2, Information technology — Document Schema Definition Language (DSDL) — Part 2: Regular-
grammar-based validation — RELAX NG
NOTE The first reference permits the use of XML and the second, RELAX NG,provides a specification for XML
modules. RELAX NG is a schema language for XML, standing for REgular LAnguage for XML for Next Generation, and
simplifies and extends the features of DTDs, Document Type Definitions.
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 8879 and ISO 19757-2 and the
following apply. This list is provided to clarify the terminology relating to feature structures used throughout
this part of ISO 24610. Terminology derived from XLM and other formal languages is not defined here.
3.1
alternation
operation on feature values (3.23) that returns one and only one of the values supplied as its argument
NOTE Given a feature specification F : a|b, where a|b denotes the alternation of a and b, F has either the value a or
the value b, but not both.
3.2
atomic value
value (3.23) without internal structure, i.e. value other than feature structure (3.10) and collection (3.4)
© ISO 2006 – All rights reserved 1

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3.3
boxed label
label in box used in a matrix notation to denote a value shared by several features (3.8)
NOTE The label may be any alphanumeric symbol.
3.4
collection
list, set, or multiset of values (3.23)
NOTE A list is an ordered collection of entities some of which may be identical. A set is an unordered collection of
unique entities. A multiset is an unordered collection of entities that may or may not be unique; it is sometimes referred to
as a bag.
3.5
complex value
value (3.23) represented either as a feature structure (3.10) or as collection (3.4)
3.6
concatenation
operation of combining two lists of values (3.23) into a single list
3.7
empty feature structure
feature structure (3.10) containing no feature specifications (3.9)
3.8
feature
property of an entity
NOTE The combination of feature and feature-value constitutes a feature specification (3.9). For example, number
is a feature, singular is a value, and a pair is a feature specification.
3.9
feature specification
assignment of a value (3.23) to a feature (3.8)
NOTE Formally, it is treated as a pair of a feature and its value.
3.10
feature structure
set of feature specifications (3.9)
NOTE The minimum feature structure is the empty feature structure (3.7).
3.11
graph notation
notation of feature structure (3.10) in a single rooted graph
3.12
incompatibility
relation between two feature structures (3.10) which have conflicting types (3.19) or at least one common
feature (3.8) with incompatible values (3.23)
NOTE Two feature structures that are incompatible cannot be unified. The empty feature structure (3.7) is
compatible with any other feature structure.
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3.13
matrix notation
attribute-value matrix
AVM
notation that uses square brackets to represent feature structures (3.10)
NOTE In a matrix notation, each row represents a feature specification (3.9), with the feature name and the feature
value separated by a colon (:), space ( ) or the equals sign (=).
3.14
merge
generic operation that includes union (3.22) of sets or multisets and concatenation (3.6) of lists
3.15
negation
(unary) operation on a value (3.23) denoting any other value incompatible with it
NOTE In this part of ISO 24610, negation applies to values only and is not understood as a truth function as in
ordinary bivalent logics.
3.16
path
sequence of labeled arcs connecting nodes in a graph
3.17
structure sharing
re-entrancy
relation between two or more features (3.8) within a feature structure (3.10) that share a value (3.23)
3.18
subsumption
relationship between two feature structures (3.10) in which one is more specific than the other
NOTE A feature structure A is said to subsume a feature structure B if A is at least as informative as B. Subsumption
is a reflexive, antisymmetric, and transitive relation between two feature structures.
3.19
type
name of a class of entities
NOTE Feature structures (3.10) may be characterized by grouping them into certain classes. Types are used to
name such classes.
3.20
typed feature structure
feature structure (3.10) labelled by a type (3.19)
NOTE In the graph notation (3.11), each node is labelled with a type. In the matrix notation (3.13), a type is
ordinarily placed at the upper left corner of the inside of the pair of square brackets that represents a typed feature
structure. In XML notation, the type is supplied as the value (3.23) of a type attribute on the element.
3.21
unification
operation that combines two compatible feature structures (3.10) into the least informative feature structure
that contains the information from the two
3.22
union
operation that combines two sets, or multisets, into one
NOTE The corresponding operation for lists is concatenation (3.6).
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3.23
value
information about an entity
NOTE There are two kinds of feature values: atomic value (3.2) and complex value (3.5).
4 General characteristics of feature structure
4.1 Overview
A feature structure is a general purpose data structure that identifies and groups together individual features
by assigning a particular value to each. Because of the generality of feature structures, they can be used to
represent many different kinds of information. Interrelations among various pieces of information and their
instantiation in markup provide a meta-language for representing analysis and interpretation of linguistic
content. Moreover, this instantiation allows a specification of a set of features with values of specific types and
1)
restrictions, by means of feature system declarations, or other XML mechanisms discussed in ISO 24610-2 .
4.2 Use of feature structures
Feature structures provide partial information about an object by specifying values for some or all of its
features. For example, if a female employee named Sandy Jones who is 30 years old is of the present
concern, then that person’s sex, name and age can be specified in a succinct manner by assigning a value to
each of these three features. These pieces of information can be put into a simple set notation, as in:
(1) Employee
{, , }
Feature structures are generally used as a vehicle for linguistic descriptions. For example, the phoneme /p/ in
English can be analysed in terms of its distinctive features: consonantal, anterior, voiceless, non-continuant or
stop sound segment, etc. Each of these features may be combined with one or other of the binary values
plus(+) and minus(-) to provide a feature specification. In a phonemic analysis such as the following, the value
of a feature specifies the presence or absence of that feature:
(2) Sound segment /p/
{, , , }
In such an analysis, the sound segment /p/ is distinguished from other phonemes in terms of the presence or
absence of specific features. For example, /p/ differs from the phoneme /b/ in VOICING, and from /k/ in
articulatory position: one is articulated at the anterior, (the lip or alveolar area of the mouth), and the other at
the nonanterior, namely the back of the oral cavity.
This feature analysis can be extended to other kinds of description. Consider a verb like “love”. Its features
include both syntactic and semantic properties: as a transitive verb, it takes an object as well as a subject as
its arguments, expressing the semantic relation of loving between two persons or animate beings. The exact
representation of these feature specifications requires a detailed elaboration of what feature structures are.
For now, these grammatical features can be roughly represented in a set format like the following:
(3) Grammatical features of the verb “love”
{, , },
where POS stands for part of speech.

1) Under preparation.
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Since its first extensive use in generative phonology in mid-60s, the feature structure formalism has become
an essential tool not only for phonology, but more generally in support of syntax and semantics as well as
lexicon building, especially in computational work. Feature structures are used to describe and model
linguistic entities and phenomena by specifying their properties. In the next clauses, some of the formal
properties of feature structures are outlined together with means of representing them in a systematic manner.
4.3 Basic concepts
Feature structures may be viewed in a variety of ways. The most common and perhaps the most intuitive
views are the following:
⎯ a set of feature specifications that consists of pairs of features and their values;
⎯ labelled directed graphs with a single root where each arc is labelled with the name of a feature and
directed to its value.
In set-theoretic terms, a feature structure FS can be defined as a partial function from a set Feat of features to
a set FeatVal of values, where FeatVal consists of a set AtomVal of atomic values and a set FS of feature
structures.
(4) A feature structure as a set or partial function
FS ⊆ {⎥F∈Feat,v∈FeatVal}
i i i i
or
FS : Feat ⎯→FeatVal
where FeatVal = AtomVal∪ FS and where FeatVal stands for all possible values.
Values may be regarded as either atomic or complex. Atomic values are entities without internal structure,
while complex values may be feature structures or
...