Dependency Grammar - Dependency Vs. Constituency

Dependency Vs. Constituency

Dependency is a one-to-one correspondence: for every element (e.g. word or morph) in the sentence, there is exactly one node in the structure of that sentence that corresponds to that element. The result of this one-to-one correspondence is that dependency grammars are word (or morph) grammars. All that exist are the elements and the dependencies that connect the elements into a structure. This situation should be compared with the constituency relation of phrase structure grammars. Constituency is a one-to-one-or-more correspondence, which means that given a sentence, for every element in that sentence, there are one or more nodes in the structure that correspond to that element. The result of this difference is that dependency structures are minimal compared to their constituency structure counterparts, since they tend to contain many fewer nodes.

These two trees illustrate just two possible ways to render the dependency and constituency relations (see below). The dependency tree is an "ordered" tree, i.e. it reflects actual word order. Many dependency trees abstract away from linear order and focus just on hierarchical order, which means they do not show actual word order. The constituency tree follows the conventions of bare phrase structure (BPS), whereby the words themselves are employed as the node labels.

The distinction between dependency- and constituency-based grammars derives in a large part from the initial division of the clause. The constituency relation derives from an initial binary division, whereby the clause is split into a subject noun phrase (NP) and a predicate verb phrase (VP). This division is certainly present in the basic analysis of the clause that we find in the works of, for instance, Leonard Bloomfield and Noam Chomsky. Tesnière, however, argued vehemently against this binary division, preferring instead to position the verb as the root of all clause structure. Tesnière's stance was that the subject-predicate division stems from term logic and has no place in linguistics. The importance of this distinction is that if one acknowledges the initial subject-predicate division in syntax as something real, then one is likely to go down the path of constituency grammar, whereas if one rejects this division, then the only alternative is to position the verb as the root of all structure, which means one has chosen the path of dependency grammar.