Dependency Syntax and Universal Dependencies › Universal Dependencies

Universal Dependencies provides a shared framework for cross-linguistic annotation. Its design principles and file format let you interpret a treebank using common categories, while its language-specific guidance records how those categories apply to particular constructions.

Here, "universal" refers to the shared annotation categories. UD does not claim that all languages have the same syntax. It aims to describe the syntax of different languages using comparable categories. Shared categories provide a basis for cross-lingual training and evaluation; consistent application and annotation coverage still matter.

UD combines treebanks with annotation guidelines and common inventories for parts of speech, morphological features, and dependency relations. This chapter uses the version 2 guidelines. The official introduction explains the project's goals, including multilingual parsing and language analysis. Counts of languages and treebanks change with releases and are not needed to understand the representation.

Cross-linguistic consistency is a design objective, not a claim that all languages share identical syntax. Language-specific documentation is part of using a treebank correctly. A relation called obj should be read together with the universal definition, the language's grammatical facts, and the treebank's annotation decisions.

The UD guidelines give annotators shared principles. The most important ones for understanding this representation are:

• Typologically universal. UD aims to use the same relation name (nsubj, obj, amod) for the same grammatical function across languages, even when that function is expressed through different morphology. A separate adposition or case clitic can be a case dependent; a bound genitive suffix is represented in a word's features, not as a separate dependency node.
• Shallow representation. UD annotates surface syntax, not deep structure. There is no movement, no traces, and no abstract empty categories in the basic layer. Enhanced dependencies can record additional relations that the basic tree leaves implicit.
• Explicit annotation commitments. UD uses a dependency-based, lexicalist framework with stated head-selection and relation definitions. Comparable annotation requires applying those shared conventions.
• Surface-oriented. The annotation reflects what is actually written or spoken, not what would be underlying. Disfluencies, typos, and unusual constructions are annotated as they appear.
• Content-headed by default. Content words normally anchor the syntactic backbone and related function words depend on them. The exceptions described earlier remain part of the scheme.

The basic tree records word-level syntactic relations. The enhanced layer can add links for constructions such as control and ellipsis. An application still needs to decide whether those layers supply the information it requires.

UD defines a closed set of 17 universal part-of-speech tags. Every treebank uses these tags for the UPOS column:

These tags are coarse by design. They are chosen so that the same tag applies across languages even when the morphology is radically different. The optional XPOS column holds a language- or treebank-specific tag, sometimes retained from a source annotation scheme. English treebanks may use Penn Treebank tags and German treebanks may use the Stuttgart-Tübingen Tagset; a treebank's documentation specifies its choice. XPOS is _ when unavailable. The two columns serve different purposes: UPOS supports cross-lingual comparison, while XPOS can preserve finer distinctions from a particular annotation scheme.

UD also standardizes the FEATS column, which holds morphological features as key-value pairs. Features describe word-level morphology. Examples you will encounter include:

• Nominal features: Case, Gender, Number, Person, Definite, PronType, Animacy.
• Verbal features: Tense, Aspect, Mood, Voice, VerbForm, Polarity, Evident.

A typical English noun might carry Number=Sing, while a German noun might carry Case=Acc|Gender=Masc|Number=Sing. The shared feature inventory makes these bundles easier to compare, while language-specific definitions and annotation coverage still matter.

The features make the word-level comparison more explicit. English dog and German Hund have identical nsubj relations in structurally parallel sentences, but the German noun may carry case information the English one lacks. A parser can use annotated features as inputs when its configuration supports them. However, a case value alone does not determine a dependency relation across languages, and no transfer-performance claim follows from these two illustrative bundles.

The DEPS column can carry an optional enhanced dependency graph. Basic trees are obligatory in UD treebanks; enhanced annotation is not available in every treebank. The enhanced graph adds arcs that are absent from the basic tree because of control, ellipsis, relative clauses, and similar phenomena.

Some of the constructions the enhanced layer handles:

• Relative clauses. In the dog which was a Terrier, the basic tree has acl:relcl from dog to the clause root, and which is nsubj inside the clause. In the detailed enhanced analysis, dog has a ref arc to which, and dog replaces which as the nsubj dependent of Terrier. This makes the antecedent's role inside the relative clause explicit.
• Control and raising. In She wants to leave, the basic tree has leave as xcomp of wants. The current detailed enhanced guidelines add an arc from leave to She labelled nsubj:xsubj, marking an externally supplied subject. Older annotations and the general syntax overview may use bare nsubj, so check the treebank's convention.
• Gapping. In She ate fish and he pasta, the basic tree connects the overt words using the ellipsis conventions. If the treebank supplies this enhancement, the enhanced graph can add an empty node for the elided predicate and its argument links.
• Shared coordination arguments. An argument that depends on the first conjunct in the basic tree can receive an additional relation to another conjunct in the enhanced graph.

Enhanced dependencies can make useful interpretation links explicit, but they require additional annotation decisions. They may add edges, alter a basic relation, and introduce empty nodes for ellipsis. The result is not necessarily a tree or even a strict supergraph of the basic tree. Which enhancement types are supplied must be checked in the treebank documentation. The enhanced syntax guidelines describe these constructions.