Dependency grammar (DG) is a class of modern syntactic theories that are all based on the dependency relation and that can be traced back primarily to the work of Lucien Tesnière. Dependency grammars are distinct from phrase structure grammars (=constituency grammars), since they lack phrasal nodes. Structure is determined by the relation between a word (a head) and its dependents. Dependency structures are flatter than constituency structures in part because they lack a finite verb phrase constituent, and they are thus well suited to languages with free word order, such as Czech and Turkish.
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The notion of dependencies between grammatical units has existed since the earliest recorded grammars, e.g. Panini, and the dependency concept therefore arguably outdates the constituency notion by many centuries.[1] Ibn Mada was the first grammarian to use the term dependency in the grammatical sense that we use it today. He was born in 1119 in Cordoba, studied in Sevilla and Ceuta, and died 1195 in Sevilla. In early modern times, the dependency concept seems to have coexisted side by side with the constituency concept, the latter having entered Latin, French, English and other grammars from the widespread study of term logic of antiquity.[2]
Modern dependency grammars, however, begin primarily with the work of Lucien Tesnière. Tesnière was a Frenchman, a polyglot, and a professor of linguistics at the universities in Strasbourg and Montpellier. His major work Éléments de syntaxe structurale was published posthumously in 1959 - he died in 1954. The basic approach to syntax he developed seems to have been seized upon independently by others in the 1960s[3] and a number of other dependency-based grammars have gained prominence since those early works.[4] Dependency grammar has generated a lot of interest in Germany[5] in both theoretical syntax and language pedagogy. In recent years, the great development surrounding dependency-based theories has come from computational linguistics. Dependency-based systems are increasingly being used to parse natural language and generate tree banks. Interest in dependency grammar is growing at present, the first international conference on dependency linguistics having taken place just recently (Depling 2011).
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[6] 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. 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.[7] 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.
Tesnière emphasized that syntactic units are organized along two dimensions,[8] the horizontal (precedence) dimension and the vertical (dominance) dimension. In this respect, acknowledging dependency-based structures encourages one to isolate units in the one or the other dimension. Combinations of elements that are organized along the horizontal dimension alone are called strings (a common concept) and combinations of elements organized along the vertical dimension alone are called catenae (a novel concept).
The following frameworks are dependency-based:
Link grammar is also based on the dependency relation, but link grammar does not include directionality in the dependencies between words, and thus does not describe head-dependent relationships. Hybrid dependency/constituency grammar uses dependencies between words, but also includes dependencies between phrasal nodes - see for example, the Quranic Arabic Dependency Treebank. The derivation trees of Tree-Adjoining Grammar are dependency-based, although the full trees of TAG are constituency-based, so in this regard, it is not clear whether TAG should be viewed more as a dependency or constituency grammar.
There are major differences between the grammars just listed. In this regard, the dependency relation is compatible with other major tenets of theories of grammar. Thus like constituency grammars, dependency grammars can be mono- or multistratal, representational or derivational, construction- or rule-based.