A controlled vocabulary is an organized arrangement of words and
phrases used to index content and/or to retrieve content through
browsing or searching. It typically includes preferred and variant terms
and has a defined scope or describes a specific domain.
2.1. Purpose of Controlled Vocabularies
The purpose of controlled vocabularies is to organize information and
to provide terminology to catalog and retrieve information. While
capturing the richness of variant terms, controlled vocabularies also
promote consistency in preferred terms and the assignment of the same
terms to similar content.
Given that a shared goal of the cultural heritage community
is to improve access to visual arts and material culture information,
controlled vocabularies are essential. They are necessary at the indexing
phase because without them catalogers will not consistently use the same
term to refer to the same person, place, or thing. In the retrieval process,
various end users may use different synonyms or more generic terms to
refer to a given concept. End users are often not specialists and thus need
to be guided because they may not know the correct term.
The most important functions of a controlled vocabulary are
to gather together variant terms and synonyms for concepts and to link
concepts in a logical order or sort them into categories. Are a rose window and a Catherine wheel the same thing? How is pot-metal glass related
to the more general term stained glass? The links and relationships in
a controlled vocabulary ensure that these connections are defined and
maintained, for both cataloging and retrieval.
2.2. Display Information and Controlled Information
Records for cultural objects typically contain both descriptive data
and administrative data, which are outlined and defined in CCO and
CDWA. Data elements record an identification of the type of object,
creation information, dates of creation, place of origin and current location,
subject matter, and physical description, as well as administrative
information about provenance, history, acquisition, conservation, context
related to other objects, and the published sources of this information.
Both descriptive and administrative data must be maintained in
ways that will accommodate two categories of information: information
intended for display to end users and information intended for retrieval.
Information utilized for retrieval should be adapted for controlled vocabularies
and controlled format.
Why are the display and indexing of information separate
issues? Art and cultural heritage information provides unique challenges
in display and retrieval. Information must be displayed to users
in a way that allows expression of nuance, ambiguity, and uncertainty.
The facts about cultural objects and their creators are not always known
or straightforward, and it is misleading and contrary to the tenets of
scholarship to fail to express this uncertainty. At the same time, efficient
retrieval requires indexing according to consistent, well-defined rules and
A successful catalog of art and cultural heritage information
maintains a balance between flexible standards and consistent rules. On
the one hand, it must be flexible in allowing the expression of uncertainty
and ambiguity where the discipline requires it, while also accommodating
nuance and differences in style between departments and
institutions. On the other hand, it must apply rules consistently where it
is most critical—namely, for information that is indexed for retrieval.
In the context of this book, the controlled fields in a record are
specially formatted and often linked to controlled vocabularies (authorities),
controlled lists, or ruled by formatting restrictions (e.g., formatting
of numbers) to allow for successful retrieval.
For a full list of fields for art information and their requirements
for free-text, controlled format, or controlled vocabulary, see
CDWA (fields and rules) and CCO (detailed rules for a subset of
the CDWA categories).
2.2.1. Display Information with Controlled Vocabularies
It is often necessary to allow fuzziness in the expression of information
that at the same time must be retrievable via terminology from
a controlled vocabulary; in certain key areas of a work record, this is
accomplished by including separate display and indexing fields for
the same information. For example, in the creation statement and in
technique, medium, and support statements, the information may be
complex and may include indications of uncertainty through the use of
words such as or or probably.
The most effective way to express the nuances of such information
is to use natural language in a display field and to index the same
information separately, using controlled vocabulary (typically contained
in an authority file). In the examples below, the creator's role is indexed
with controlled terms, and the identity of the creator is indexed as well.
The Creator Description field is free text, and authority files control
the other fields. See Chapter 6: Local Authorities for a discussion of
authority files and local authorities.
Creator Description: Vincent van Gogh (Dutch, 1853–1890)
Role: painter Identity: Gogh, Vincent van
Creator Description: Marco Ricci (Venetian, 1676–1730),
figures by Sebastiano Ricci (Venetian, 1659–1734)
Role: painter Extent: landscape | architecture
Identity: Ricci, Marco
Role: painter Extent: figures Identity: Ricci, Sebastiano
Creator Description: primary painter and calligrapher was
Dai Xi (Chinese, 1801–1860), with additional inscriptions
and colophons added by other officials; commissioned by Wu
Roles: painter | calligrapher Identity: Dai Xi
Role: patron Identity: Wu Zhongzhun
2.2.2. Controlled Vocabularies vs. Controlled Format
While controlled vocabularies are organized sets of controlled terminology
values (often with other information as well), the term controlled
format refers to rules concerning the allowable data types and formatting
of information. Fields may have controlled format in addition to being
linked to controlled vocabulary, or the controlled format may exist in the
absence of any finite controlled list of acceptable values.
Controlled format may govern the expression of Unicode or
other characters in either a free-text field or in a field that is linked to a
controlled vocabulary. Controlled format is also suitable for recording
measurements, geographic coordinates, and other information in fields
where numbers or codes are used. Restrictions may be placed on the field
in order to regulate the number of digits allowed, the expression of decimals
and negative numbers, and so on, ideally in compliance with ISO,
NISO, or another appropriate standard where possible.
The examples below juxtapose a set of materials fields that use
display and controlled vocabulary fields with a set of measurements fields.
Fields such as Role and Material Name contain controlled vocabulary.
However, in the measurements fields, the numbers in Value are indexed
with controlled format but not controlled vocabulary.
Materials/Techniques Description: egg-tempera paint with
tooled gold-leaf halos on panel
Role: medium Material Name: egg tempera | gold leaf
Role: support Material Name: wood panel
Technique Name: painting | gold tooling
Dimensions Description: comprises 10 panels; overall: 280 x
215 x 17 cm (110 1⁄4 x
84 5⁄8 x 6 3⁄4)
Value: 10 Type: count
Value: 280 Unit: cm Type: height
Value: 215 Unit: cm Type: width
Value: 17 Unit: cm Type: depth
Controlled format is also typically used for dates, such as
the date of discovery or date of creation of an artwork. For such dates,
controlled fields may be used in combination with a Display Date field.
The issues involved in recording data about dates illustrate the
necessity of displaying information in a way that accurately expresses
nuance and ambiguity to the end user, while at the same time formatting
the dates consistently to allow retrieval. A free-text field for a Display
Date can be used to express complex concepts and nuance, as in the
Display Creation Date: probably 1711
Display Creation Date: ca. 1910–ca. 1915
Display Creation Date: designed in the 1470s, constructed
The Display Date field should be combined with controlled
Earliest Date and Latest Date fields that contain beginning and ending
limits to enable searches on spans of time. The cataloger may estimate
Earliest and Latest dates to allow for the leeway required by expressions
such as ca., before, or probably.
The controlled Earliest and Latest fields do not contain
controlled vocabulary per se, but they require a controlled format in
which only numbers are allowed. A minus sign can be used to express
dates BCE (Before Current Era) as negative numbers; dates CE (Current
Era) are positive numbers. A rule should be in place ensuring that the
latest date is always greater than or equal to the earliest date.
Display Creation Date: ca. 1913
Earliest: 1908 Latest: 1918
Display Creation Date: constructed 286–199 BCE
Earliest: –286 Latest: –199
Display Creation Date: 12th century
Earliest: 1100 Latest: 1199
Display Creation Date: Middle Minoan Palace period,
ca. 1600 BCE
Earliest: –1630 Latest: –1570
Display Creation Date: 1039 anno Hegirae (1630 CE)
Earliest: 1630 Latest: 1630
Date fields should typically be controlled through locally
defined rules rather than with default rules contained in the system.
Although most systems promote the use of a special data type called date
with predefined rules, this standard date data type does not generally
work because art information requires the expression of dates up to many
thousands of years BCE, and standard date data types are intended only
for more modern dates (e.g., allowing 8-byte integers that represent dates
ranging from 1 January of the year 0001 through 31 December of the
2.3. Types of Controlled Vocabularies
Most controlled vocabularies discussed in this book are structured vocabularies.
A structured vocabulary emphasizes relationships between and
among the concepts represented by the terms or names in a vocabulary.
2.3.1. Relationships in General
In the context of this book, the term relationship means a state of
connectedness or an association between two things in a database—in
this case, fields or tables in a database for a controlled vocabulary.
One important type of relationship is between equivalents; for
example, Harlem Renaissance and New Negro Renaissance refer to the same
cultural movement that flourished in New York City in the 1920s.
Other relationships in a structured vocabulary include links
that organize terms and provide context; for example, when discussing
architectural drawings, an orthographic projection is a type of (child of)
parallel projection and a sibling of axonometric projection, all of which are
organized under processes and techniques.
The most common types of controlled vocabularies used for art
and architecture include subject heading lists, simple controlled lists,
synonym ring lists, taxonomies, and thesauri. Many of the definitions below are based on the discussions in ANSI/NISO Z39.19-2005: Guidelines
for the Construction, Format, and Management of Monolingual
Controlled Vocabularies and the related international standard, ISO
2788:1986: Documentation—Guidelines for the Establishment and
of Monolingual Thesauri. Note that the types of vocabularies
described below are not always mutually exclusive; for example, a single
vocabulary can be both a thesaurus and an authority.
2.3.2. Subject Heading Lists
Subject headings, or simply headings, are uniform words or phrases
intended to be assigned to books, articles, or other documents in order
to describe the subject or topic of the texts and to group them with texts
having similar subjects. The most commonly used subject headings in
libraries in the United States are the Library of Congress Subject Headings
(LCSH), which form a comprehensive list of preferred terms or strings,
often with cross-references. Another well-known set of subject headings is
the Medical Subject Headings (MeSH), which is used for indexing journal
articles and books on medical science. MeSH incorporates a thesaurus
structure with subject headings.
Subject heading lists are typically arranged in alphabetical
order, with cross-references between the preferred, nonpreferred, and
other related headings. This emphasis on a preferred entry and links
to synonyms may be found in other types of authorities. However,
subject headings differ from the other vocabularies discussed here in the
following fundamental way: precoordination of terminology is a characteristic
of subject headings in that they combine several unique concepts
together in a string. For example, the heading Medieval bronze vessels
combines a period, a material, and a work type in one heading.
Subject heading lists typically include separate listings of standardized
subheadings (e.g., geographic locations) that may be combined
with designated headings according to prescribed rules. Various styles
of subject heading displays are included in the examples below. LCSH
displays two dashes and parentheses or periods as required, while other
styles may omit punctuation or use colons or em dashes for compound
phrases. In LCSH, MeSH, and other authorities, the parts of a compound
heading may be stored in separate MARC format subfields to allow variations
in displays as desired.
Bicycle racing--United States
Cat family (Mammals)--Literary collections
South Africa. Arts and Culture Task Group
Film history: Movements and styles
Embryonic and Fetal Development
Medieval bronze vessels
Great Britain Description and travel 1801–1900
18.104.22.168. Other Headings
Other types of headings or labels may be used to uniquely identify or
disambiguate one vocabulary entry from another. That is, the vocabulary
record itself represents a single unique person, place, or thing, but
its name is displayed with information in addition to the name. For
example, the name of a creator may be listed with a short biographical
string (e.g., Flemish painter, 1423–1549) to form a heading or label for
display in a work record. This type of heading or label is discussed in
Chapter 7: Constructing a Vocabulary or Authority.
2.3.3. Controlled Lists
A controlled list is a simple list of terms used to control terminology.
In a well-constructed controlled list, the following is true: each term is
unique; terms are not overlapping in meaning; terms are all members
of the same class (i.e., having the same level of rank in a classification
system); terms are equal in granularity or specificity; and terms
are arranged alphabetically or in another logical order. These lists are
also called flat term lists or pick lists, referring to the typical method of
their implementation in an information system. Where appropriate,
controlled lists should be derived from larger published standard
Controlled lists are usually designed for a very specific database
or situation and may not have utility outside that context. They are best
employed in certain fields of a database where a short list of values is
appropriate and where terms are unlikely to have synonyms or ancillary
information. However, as with any vocabulary for cataloging, it is preferable
that definitions of the terms be made available to ensure consistency
among catalogers. Below is an example of a controlled list for the Classification
field in a work record.
The advantage of such lists is that the cataloger or indexer has
only a short list of terms from which to choose, thus ensuring more
consistency and reducing the likelihood of error. In addition to the Classification
field, examples of other art information fields that may benefit
from a simple controlled list are Title Type (e.g., artist's, descriptive,
inscribed, etc.), Title Language (e.g., English, French, German, Italian,
Spanish, etc.), or Title Preference (e.g., preferred, alternate). Dozens of
areas of a work record may be better suited for a short controlled list
rather than a more complex controlled vocabulary. From the end-user
perspective, such short lists may be easier to navigate than more complex
lists, particularly for nonspecialist users.
2.3.4. Synonym Ring Lists
A synonym ring is a simple set of terms that are considered equivalent
for the purpose of retrieval. Equivalence relationships in most controlled
vocabularies should be made only between terms and names that have
genuine synonymy or identical meanings. However, synonym rings are
different. Even though they are classified as controlled vocabularies, they
are almost always used in retrieval rather than indexing. They are used
specifically to broaden retrieval (this is often referred to as query expansion):
thus, synonym rings may in fact contain near-synonyms that have
similar or related meanings, rather than restricting themselves to only
terms with true synonymy.
Typically, synonym rings occur as sets of flat lists and are used
behind the scenes of an electronic information system. They are most
useful for providing access to content that is represented in texts and
other instances of natural, uncontrolled language.
Even though catalogers do not use synonym rings for indexing,
subject experts should be involved in the creation of synonym rings for
retrieval. The most successful synonym rings are constructed manually by
subject matter experts who are also familiar with the specific content of
the information system, user expectations, and likely searches.
In the example below, synonym rings (each represented in an
individual row) represent true synonyms as well as more generic terms
and other terms that are related within the specific context of a given text.
The example could represent a partial synonym ring list for a text about
art depicting certain migrating birds. If a user enters crows, the search
mechanism returns any text containing birds or any of the other terms
in the same synonym ring as crows. Even though these terms are not
synonyms, the implementer has judged that these links make sense for
broad retrieval in this particular text. Other automated retrieval strategies
may be in place as well; for example, the search algorithm may automatically
truncate the s to allow matches in English on both singular and
birds, avian, storks, crows, ravens, herons, Ciconiidae, Corvus,
migration, nonmigratory, migratory, travel, flying, altitude
clouds, cumulus, nimbus, storm clouds, cloudy
wind, windy, windstorm, wind damage, air flow, jet stream
2.3.5. Authority Files
An authority file is a set of established names or headings and cross-references
to the preferred form from variant or alternate forms. Illustrated on
the following page is the LCNAF—the Library of Congress/NACO (Name
Authority Cooperative Program) Authority File—an authority widely used
in libraries in North America.
Common types of authority files are name authority files and
subject heading authority files. However, any listing of terms, names, or
headings that distinguishes between a preferred term, name, or heading
and alternate or variant names may be used as an authority. In other
words, almost any type of controlled vocabulary—with the exception of
a synonym ring list—may be used as an authority.
Authority control refers as much to the methodology as to a
particular controlled vocabulary. If a controlled vocabulary is accepted by
a given community as authoritative, and if it is used in order to provide
consistency in data, it is being used as an authority. A local authority file
is often compiled from terminology from one or more published standard
controlled vocabularies. The establishment of local authorities is
discussed in Chapter 6: Local Authorities.
A taxonomy is an orderly classification for a defined domain. It may also
be known as a faceted vocabulary. It comprises controlled vocabulary
terms (generally only preferred terms) organized into a hierarchical structure.
Each term in a taxonomy is in one or more parent/child (broader/
narrower) relationships to other terms in the taxonomy. There can be
different types of parent/child relationships, such as whole/part, genus/
species, or instance relationships. However, in good practice, all children
of a given parent share the same type of relationship.
A taxonomy may differ from a thesaurus in that it generally
has shallower hierarchies and a less complicated structure. For example,
it often has no equivalent (synonyms or variant terms) or related terms
(associative relationships). The scientific classifications of animals and
plants are well-known examples of taxonomies. A partial display of Flavobacteria in the taxonomy of the U.S. National Center for Biotechnology
Information is above.
In common usage, the term taxonomy may also refer to any classification
or placement of terms or headings into categories, particularly a
controlled vocabulary used as a navigation structure for a Web site.
2.3.7. Alphanumeric Classification Schemes
Alphanumeric classification schemes are controlled codes (letters or
numbers, or both letters and numbers) that represent concepts or headings.
They generally have an implied taxonomy that can be surmised
from the codes. The Dewey Decimal Classification (DDC) system is an
example of a numeric classification scheme with which many people are
familiar, given that it is one of the two major systems used in libraries
in the United States (the other is the Library of Congress Classification
[LCC] system). In the Dewey system, the universe of knowledge is
divided into sets of three-digit numbers. The arts are represented in the
700-number series; sculpture is represented by numbers between 730
and 739. For example, the number 735 has been established to indicate sculpture after the year 1400 CE. To that number may be added additional
decimal indicators to further specify the topic by geographic or
other categories. For example, 735.942 refers to sculpture dating after
1400 in England, because the extension 9 indicates geographic area,
4 indicates Europe, and 2 indicates England.
An alphanumeric classification scheme used for the iconography
of art is Iconclass, discussed in Chapter 4: Vocabularies for Cultural
A thesaurus combines the characteristics of synonym ring lists and
taxonomies, together with additional features. A thesaurus is a semantic
network of unique concepts, including relationships between synonyms,
broader and narrower (parent/child) contexts, and other related concepts.
Thesauri may be monolingual or multilingual. Thesauri may contain
three types of relationships: equivalence (synonym), hierarchical (whole/
part, genus/species, or instance), and associative.
Thesauri may also include additional peripheral or explanatory
information about a concept, including a definition (or scope note),
bibliographic citations, and so on. A thesaurus is more complex than a
simple list, synonym ring list, or simple taxonomy. Thesauri employ the
versatile and powerful vocabulary control generally recommended for use
as authorities in databases relating to art and cultural heritage.
The primary type of vocabulary discussed in this book is a
thesaurus. Thesauri that contain art terminology include the Getty vocabularies,
Chenhall's Nomenclature, and the TGM, which are discussed in
Chapter 4: Vocabularies for Cultural Objects.
The term thesaurus may also be used for any controlled vocabulary
arranged in a known order, displayed with standardized relationship
indicators, and generally used for browsing in postcoordinated information
storage and retrieval systems.
Whereas the vocabularies discussed above are the ones most commonly
used for art information, discussions of controlled vocabularies may also
In common usage in computer science, an ontology is a formal,
machine-readable specification of a conceptual model in which concepts,
properties, relationships, functions, constraints, and axioms are all
explicitly defined. Such an ontology is not a controlled vocabulary, but
it uses one or more controlled vocabularies for a defined domain and
expresses the vocabulary in a representative language that has a grammar for using vocabulary terms to express something meaningful. Ontologies generally divide the realm of knowledge that they represent into the
following areas: individuals, classes, attributes, relations, and events.
The grammar of the ontology links these areas together by formal
constraints that determine how the vocabulary terms or phrases may be
used together. There are several grammars or languages for ontologies,
both proprietary and standards-based. An ontology is used to make
queries and assertions.
Ontologies have some characteristics in common with faceted
taxonomies and thesauri, but ontologies use strict semantic relationships
among terms and attributes with the goal of knowledge representation
in machine-readable form, whereas thesauri provide tools for cataloging
Ontologies are used in the Semantic Web, artificial intelligence,
software engineering, and information architecture as a form of
knowledge representation in electronic form about a particular domain
In the example above, each item in the ontology belongs to
the subclass above it. Items can also belong to various other classes,
although the relationships may be different. For example, a watercolor
is a painting, but it may also be classified as a drawing because it is a
work on paper. Van Gogh's Irises could be classified with oil paintings
(with the relationship type medium is) but also with Post-Impressionist
art (with relationship type style/period is). Relationships in ontologies are
defined according to strict rules, which are different than the equivalence, hierarchical,
and associative relationships used for thesauri and other
vocabularies discussed in this book.
Folksonomy is a neologism referring to an assemblage of concepts represented
by terms and names (called tags) that are compiled through social
tagging. Social tagging is the decentralized practice and method by which
individuals and groups create, manage, and share tags (terms, names,
etc.) to annotate and categorize digital resources in an online social
environment. This method is also referred to as social classification, social
indexing, mob indexing, and folk categorization. Social tagging is not
necessarily collaborative, because the effort is typically not organized;
individuals are not actually working together or in concert, and standardization
and common vocabulary are not employed.
Folksonomies do not typically have hierarchical structure or
preferred terms for concepts, and they may not even cluster synonyms.
They are not considered authoritative because they are typically not
compiled by experts. Furthermore, they are by definition not applied to
documents by professional indexers. Given that it is impossible for the
large and varied community of creators and users of Web content to independently
add metadata in a consistent manner, folksonomies are generally
characterized by nonstandard, idiosyncratic terminology. Although
they do not support organized searching and other types of browsing
as well as tags from controlled vocabularies applied by professionals,
folksonomies can be useful in situations where controlled tagging is not
possible: they can also provide additional access points not included in
more formal vocabularies. There may be great potential for enhanced
retrieval by linking terms and names from folksonomies to more rigorously
structured controlled vocabularies.