Dependency Syntax and Universal Dependencies › Dependency Labels

A dependency tree without labels tells you which word governs which, but not how. The labels encode the grammatical role, and in UD they are drawn from a carefully standardized inventory. The head pointer gives the skeleton, and the label gives the edge its meaning. Without labels, a parser could recover the shape of the tree but not distinguish a subject from an object, or a determiner from an adjectival modifier.

The relations in this group describe the subject and objects of a predicate:

• nsubj: nominal subject. A syntactic relation rather than a semantic role: the subject can be an agent (She sleeps) or a patient (She was seen, with subtype nsubj:pass). Expletive it in It rains is instead labelled expl.
• obj: object, a nominal core argument other than the subject. The label follows the language's syntactic analysis, rather than simply numbering semantic participants.
• iobj: indirect object. A nominal core argument that is neither the subject nor the direct object. The recipient in She gave him a book is an example: him is iobj and book is obj. Recipient meaning alone does not determine this syntactic label across languages.
• csubj: clausal subject. The subject is a clause, as in That she left surprised him.
• ccomp: clausal complement. A core clausal argument, as in She said that he left. It can be finite or nonfinite; its subject is not obligatorily inherited in the manner required for xcomp.
• xcomp: open clausal complement. A core predicative complement without its own subject, whose subject interpretation is necessarily supplied by an external argument, as in She wants to leave. Nonfiniteness alone does not determine the label.

The distinction is obligatory subject inheritance, not simply whether a subject word is visibly present. A ccomp can have an unexpressed subject whose interpretation is not fixed by an argument of the higher predicate. An xcomp requires that inherited interpretation. The ccomp and xcomp guidelines give contrasts that separate control from mere omission.

UD treats function words as dependents. These relations record them:

• aux: auxiliary. A function word expressing categories such as tense, aspect, mood, voice, or evidentiality. will in she will go is an English example; the universal relation also permits nonverbal markers.
• cop: copula. is in she is a doctor.
• mark: marker. Complementizers and subordinating conjunctions. that in that she left.
• case: case marker. A case-marking element treated as a separate syntactic word, such as of in the neighbor of the man. A bound case suffix is recorded through morphology, not as its own case node.
• det: determiner. the in the dog.
• cc: coordinating conjunction. and in cats and dogs.
• clf: classifier. Mandarin numeral classifiers provide examples; the presence of numeral classifiers in a language does not by itself determine their UD relation.
• aux:pass: passive auxiliary, a subtype of aux used in passive constructions.

These labels are the direct consequence of UD's lexicalist convention. The function word is not discarded. It is attached to the content word with a label that records its grammatical contribution. That design keeps the content word as the head while still preserving the information that a determiner, auxiliary, or case marker was present.

• amod: adjectival modifier. big in big dog.
• advmod: adverbial modifier. quickly in she ran quickly.
• acl: adnominal clause. A clause modifying a noun. acl:relcl is the relative clause subtype.
• appos: appositional modifier. my brother in John, my brother, arrived.
• nmod: nominal modifier. A nominal dependent of another nominal, except when it modifies a predication. the neighbor's in the neighbor's dog (with nmod:poss in English).
• nummod: numeric modifier. three in three dogs.
• compound: compound. Yorkshire in Yorkshire Terrier.
• flat: flat structure. Names and titles where no word clearly heads the others.
• fixed: fixed multiword expression.

Several additional relations complete the examples in this chapter:

• obl marks an oblique nominal, such as a temporal or locative phrase that is not a core object.
• advcl marks an adverbial clause, such as when she arrived in she left when she arrived.
• expl marks expletive or impersonal pronouns, such as the it in it rains.
• discourse marks grammatically distinctive discourse elements, such as interjection well or filler um. An English connective adverb such as however instead receives advmod.
• vocative marks a direct address like John in John, come here.
• parataxis links juxtaposed units, often clauses, where the annotation does not use ordinary coordination or subordination. The precise construction must be checked against its guidelines.
• punct marks punctuation.
• root marks the sentence root.

These labels are part of the same inventory. A supervised parser trained to predict that full inventory needs targets for these relations alongside the argument labels.

Coordination is one of the trickiest areas of UD annotation because the analysis depends on what is shared between conjuncts:

• conj: a conjunct. In a flat coordination, the later conjuncts attach to the first conjunct with conj.
• cc: a coordinating conjunction normally attaches to the conjunct it introduces, such as dogs in cats and dogs.
• Shared dependents attach to the first conjunct in the basic representation. In They buy and sell books, books is obj of buy (the first conjunct), and the enhanced representation adds an obj arc from sell to books.

This convention can look surprising at first. A reader might expect the conjunction and to be the head of the coordination, or expect shared material to attach to the nearest conjunct. UD chooses the first conjunct as the structural anchor, which gives the basic tree a consistent shape. The enhanced layer then records the fact that the shared object also belongs semantically to the second conjunct.

UD allows subtypes of the universal relations where cross-linguistically relevant distinctions exist. Subtypes are written with a colon: nsubj:pass, obl:unmarked, obl:agent, nmod:poss, acl:relcl. All relations must be documented. Globally documented subtypes need no additional page; other subtypes require language-specific documentation and registration. Consult the treebank's documentation for the conventions it uses. The subtype system gives annotators a way to record finer distinctions without expanding the universal label set without limit.

These examples illustrate English subtypes:

• nsubj:pass: passive subject. She in She was seen.
• obl:unmarked: an adpositionless nominal oblique, as in yesterday in I saw her yesterday. In English UD, release 2.15 merged the older temporal subtype obl:tmod and non-temporal obl:npmod into this relation; older treebanks can retain those labels.
• obl:agent: agentive oblique in a passive. by the doctor in She was seen by the doctor.
• nmod:poss: possessive nominal modifier. John's in John's book.
• acl:relcl: relative clause. which was a Yorkshire Terrier in the dog which was a Yorkshire Terrier.

A subtype refines a universal relation, but the scoring protocol determines how it is evaluated. Some protocols compare full strings; others remove subtypes before scoring. Do not infer the evaluation rule from the colon notation alone. The parsing evaluation chapter examines that choice explicitly.