Relations, formerly known as "links", are labelled arcs connecting a node to another node in a UNL graph. They correspond to two-place semantic predicates holding between two Universal Words. In UNL, relations have been normally used to represent semantic cases or thematic roles (such as agent, object, instrument, etc.) between UWs. The repertoire of relations is defined in the UNL Specs and it is not open to frequent additions.
Relations are represented as two-character or three-character lower-case strings.
In the UNL framework, relations describe semantic functions between two UWs. These functions are binary and directed (from a source to a target) and are claimed to be universal. Because of their similarity in name and function to syntactic relations, it may seem that the labels used for relations are different names for special grammatical functions. This is emphatically not the case. The intention is that the labels used denote specific ideas rather than grammatical structures: the idea of “something that initiates an event,” or “agent” for example, is quite different from “grammatical subject of a sentence”, even though many times the subject of a sentence will indicate the agent of the event. The agent of an event may also appear as an adjective or noun modifier, with the preposition “by” or embedded in nouns with “er” suffixes. The whole point of the conceptual relations is to have a name for these very different grammatical structures which are conceptually quite the same. Thus, the conceptual relations used in UNL are much more abstract than the grammatical relations found in sentences.
Hierarchy of Relations
Relations are organized in a hierarchy where lower nodes subsume upper nodes. The topmost level is the relation "rel", which simply indicates that there is a relation between two elements. The following level brings four general relations: participant (ptp), for the necessary arguments (subject and complements) of verbal predicates; attribute (aoj), for the necessary arguments (subject and complement) of nominal predicates; specifier (mod), for general specifiers; and adjunct (adj), for general adjuncts, including time, location and manner.
Use of relations
Relations are always used to describe semantic dependencies between syntactic constituents. In general, the following applies:
List of relations in alphabetical order
The set of relations has been undergoing some changes throughout the history of UNL. The table below presents the list of relations according to the several UNL Specs.
|cao||co-thing with attribute|
|iof||an instance of|
|per||unit to measure object|
|smd||not semantically related|