Dependency Grammar

Dependency grammar models sentence structure through directed relations between words, in which each word (the dependent) is linked to a single head, rather than through hierarchical constituents.

Definition

Dependency grammar is a family of syntactic theories in which sentence structure consists of asymmetric binary relations linking each word to a governing head, so that structure is represented as a dependency tree over words.

Scope

This topic covers dependency-based syntax: the head-dependent relation as the basic syntactic primitive, the notion of valency, the resulting dependency trees, and the contrast with constituency. It also covers the influence of dependency representations in computational linguistics, notably the Universal Dependencies framework. It does not survey constituency-based theories, which are treated in sibling topics.

Core questions

• What is the head-dependent relation, and how does it structure a sentence?
• How does valency determine the dependents a head requires?
• How do dependency representations differ from constituency representations?
• Why are dependency structures prominent in computational parsing?

Key concepts

• head-dependent relation
• valency
• dependency tree
• actants and circumstants
• projectivity
• Universal Dependencies

Key theories

Tesnière's dependency and valency

Tesnière's foundational theory in which the verb is the structural centre of the clause and governs its actants, with valency specifying the number and kind of dependents a verb takes.

Meaning-Text and formal dependency syntax

Mel'čuk's formalisation of dependency syntax within the Meaning-Text Theory, distinguishing syntactic, morphological, and semantic dependencies and giving precise criteria for the head-dependent relation.

History

Dependency analysis has roots in the ancient and medieval grammatical tradition but was given its modern theoretical form by Tesnière (1959), who centred the clause on the verb and its valency. Mel'čuk (1988) developed a rigorous formal dependency syntax. In recent decades dependency representations have become central to natural language processing, culminating in the cross-linguistically standardised Universal Dependencies annotation scheme (de Marneffe et al. 2021).

Debates

Dependency versus constituency

Whether the basic syntactic relation is the asymmetric head-dependent link or membership in hierarchical constituents, and whether the two are notational variants or substantively different.

Key figures

• Lucien Tesnière
• Igor Mel'čuk
• Joakim Nivre
• Christopher Manning

Seminal works

• tesniere1959
• melcuk1988
• demarneffe2021

Frequently asked questions

How does a dependency tree differ from a phrase structure tree?

A phrase structure tree has internal nodes labelling phrases, with words only at the leaves, whereas a dependency tree has the words themselves as nodes, linked by labelled arcs from heads to dependents. There are no phrasal nodes.

Why is dependency grammar popular in computing?

Dependency trees are compact, link words directly, and support efficient parsing algorithms. The Universal Dependencies project provides consistent dependency annotations across many languages, making them attractive for multilingual natural language processing.