An overview of the CTS URN notation

CTS URNs are part of the CITE architecture. They provide the permanent canonical references to texts or passages of text, and are used by Canonical Text Services (CTS) to identify or retrieve passages of text.

CTS URNs are a kind of Uniform Resource Name (URN). To quote from RFC 2141,


Semantics of a CTS URN


CTS URNs organize works in text groups. Text groups have no direct parallel in FRBR, and do not have a predefined semantic range. Instead, they associate works, according to traditional citation practice, in groups with various meanings. The text group may reflect authorship (e.g., a work entitled The Adventures of Huckleberry Finn might belong to a group named “Mark Twain”), or may represent some other kind of corpus (e.g., a work numbered 1 belonging to a group named “Federalist Papers”). Within a text group, a CTS URN’s work is a conceptual entity, like the FRBR work: it is an abstract idea of the content expressed in all versions of a work, in the original language or in translation. The work may optionally be identified with increasing specificity as versions (translation or edition), or exemplars (individual physical copies). The CTS URN’s version corresponds to the “expression” in the FRBR model, while exemplars correspond to “items” in FRBR parlance.


Within the smallest citation unit (such as a paragraph or section for a prose work, or line of verse for a poem), CTS URNs can further specify a span of text with a subreference. Subreferences identify indexed substrings, or a range between an indexed pair of substrings. Because subreferences are inherently language-specific, they are only valid when the work identifier is specified to the level of a version (edition or translation), or exemplar.

Resolving CTS URNs

By “resolving CTS URNs,” we mean specifically the symmetric problems of how we determine what work a CTS URN refers to, and how we determine what URN values to use to refer to a work. (The further question of how to retrieve a passage of text referred to by a CTS URN is beyond the scope of the CTS URN’s location-independent identifiers: see instead the related topic of Canonical Text Services.)

These problems are analogous to the familiar problems of resolving internet domain names to numeric addresses, and vice versa, and it is unsurprising that CTS URNs use analogous mechanisms solve them. Like the internet domain name system, CTS URNs must guarantee that the values used to identify a work are globally unique. Like DNS, CTS URNs achieve this by delegating responsibility for managing authoritative registries. Like a top-level DNS server, CTS URNs depend ultimately on a top-level registry listing what further CTS registries are responsible for specific domains. This top-level registry is housed in the Scaife Digital Library, a durable digital repository. (Further links to SDL will be added here when they are available.)

Just as a university or business can manage domain names within its own domain name space, an organization can manage a registry of canonical identifiers for texts within its domain. So the top-level registry assigns the identifier greekLit to a registry maintained at the Center for Hellenic Studies covering ancient Greek transmitted by manuscript copying. Other registries could be added to cover specific collections of epigraphic or papyrological texts.

Syntax of a CTS URN



1.  urn name space (required: always cts)
4.  passage reference (optional)
5.  subreference (optional)

The general structure of a CTS URN is therefore


Periods separate second-level hierarchical components of the work identifier and passage reference. Within either of those components, any use of a period as a data value must be escaped.

CTS namespace


Work identifiers


textgroup, work, edition | translation, exemplar


Example: The namespace Registry identifies the CHS registry of ancient Greek transmitted by manuscript copying with the namespace greekLit; the CHS registry in turn identifies the textgroup “Homer” with the ID tlg0012and the work Iliad with the ID tlg001. A URN reference to the Iliad would therefore be expressed as


Passage citations










Subreferences identify spans within a single citation unit using indexed substrings. See an introduction to the syntax and semantics of CTS URN subreferences.