Dependency grammar
Dependency grammar is a class of modern grammatical theories that are all based on the dependency relation and that can be traced back primarily to the work of Lucien Tesnière. Dependency is the notion that linguistic units, e.g. words, are connected to each other by directed links. The verb is taken to be the structural center of clause structure. All other syntactic units are either directly or indirectly connected to the verb in terms of the directed links, which are called dependencies. Dependency grammar differs from phrase structure grammar in that while it can identify phrases it tends to overlook phrasal nodes. A dependency structure is determined by the relation between a word and its dependents. Dependency structures are flatter than phrase structures in part because they lack a finite verb phrase constituent, and they are thus well suited for the analysis of languages with free word order, such as Czech or Warlpiri.
History
The notion of dependencies between grammatical units has existed since the earliest recorded grammars, e.g. Pāṇini, and the dependency concept therefore arguably predates that of phrase structure by many centuries. Ibn Maḍāʾ, a 12th-century linguist from Córdoba, Andalusia, may have been the first grammarian to use the term dependency in the grammatical sense that we use it today. In early modern times, the dependency concept seems to have coexisted side by side with that of phrase structure, the latter having entered Latin, French, English and other grammars from the widespread study of term logic of antiquity. Dependency is also concretely present in the works of Sámuel Brassai, a Hungarian linguist, Franz Kern, a German philologist, and of Heimann Hariton Tiktin, a Romanian linguist.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 has at least partially influenced the work of others in the 1960s, although it is not clear in what way these works were inspired by other sources. A number of other dependency-based grammars have gained prominence since those early works. DG has generated a lot of interest in Germany in both theoretical syntax and language pedagogy. In recent years, the great development surrounding dependency-based theories has come from computational linguistics and is due, in part, to the influential work that David Hays did in machine translation at the RAND Corporation in the 1950s and 1960s. Dependency-based systems are increasingly being used to parse natural language and generate tree banks. Interest in dependency grammar is growing at present, international conferences on dependency linguistics being a relatively recent development.
Dependency vs. phrase structure
Dependency is a one-to-one correspondence: for every element 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 grammars. All that exist are the elements and the dependencies that connect the elements into a structure. This situation should be compared with phrase structure. Phrase structure is a one-to-one-or-more correspondence, which means that, for every element in a 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 phrase structure counterparts, since they tend to contain many fewer nodes.These trees illustrate two possible ways to render the dependency and phrase structure relations. This 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. This constituency tree follows the conventions of bare phrase structure, whereby the words themselves are employed as the node labels.
The distinction between dependency and phrase structure grammars derives in large part from the initial division of the clause. The phrase structure relation derives from an initial binary division, whereby the clause is split into a subject noun phrase and a predicate verb phrase. 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 real, then one is likely to go down the path of phrase structure grammar, while if one rejects this division, then one must consider the verb as the root of all structure, and so go down the path of dependency grammar.
Dependency grammars
The following frameworks are dependency-based:- Algebraic syntax
- Operator grammar
- Link grammar
- Functional generative description
- Lexicase
- Meaning–text theory
- Word grammar
- Universal Dependencies
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 phrase structure grammars, dependency grammars can be mono- or multistratal, representational or derivational, construction- or rule-based.
Representing dependencies
There are various conventions that DGs employ to represent dependencies. The following schemata illustrate some of these conventions:The representations in are trees, whereby the specific conventions employed in each tree vary. Solid lines are dependency edges and lightly dotted lines are projection lines. The only difference between tree and tree is that tree employs the category class to label the nodes whereas tree employs the words themselves as the node labels. Tree is a reduced tree insofar as the string of words below and projection lines are deemed unnecessary and are hence omitted. Tree abstracts away from linear order and reflects just hierarchical order. The arrow arcs in are an alternative convention used to show dependencies and are favored by Word Grammar. The brackets in are seldom used, but are nevertheless quite capable of reflecting the dependency hierarchy; dependents appear enclosed in more brackets than their heads. And finally, the indentations like those in are another convention that is sometimes employed to indicate the hierarchy of words. Dependents are placed underneath their heads and indented. Like tree, the indentations in abstract away from linear order.
The point to these conventions is that they are just that, namely conventions. They do not influence the basic commitment to dependency as the relation that is grouping syntactic units.
Types of dependencies
The dependency representations above show syntactic dependencies. Indeed, most work in dependency grammar focuses on syntactic dependencies. Syntactic dependencies are, however, just one of three or four types of dependencies. Meaning–text theory, for instance, emphasizes the role of semantic and morphological dependencies in addition to syntactic dependencies. A fourth type, prosodic dependencies, can also be acknowledged. Distinguishing between these types of dependencies can be important, in part because if one fails to do so, the likelihood that semantic, morphological, and/or prosodic dependencies will be mistaken for syntactic dependencies is great. The following four subsections briefly sketch each of these dependency types. During the discussion, the existence of syntactic dependencies is taken for granted and used as an orientation point for establishing the nature of the other three dependency types.Semantic dependencies
Semantic dependencies are understood in terms of predicates and their arguments. The arguments of a predicate are semantically dependent on that predicate. Often, semantic dependencies overlap with and point in the same direction as syntactic dependencies. At times, however, semantic dependencies can point in the opposite direction of syntactic dependencies, or they can be entirely independent of syntactic dependencies. The hierarchy of words in the following examples show standard syntactic dependencies, whereas the arrows indicate semantic dependencies:The two arguments Sam and Sally in tree are dependent on the predicate likes, whereby these arguments are also syntactically dependent on likes. What this means is that the semantic and syntactic dependencies overlap and point in the same direction. Attributive adjectives, however, are predicates that take their head noun as their argument, hence big is a predicate in tree that takes bones as its one argument; the semantic dependency points up the tree and therefore runs counter to the syntactic dependency. A similar situation obtains in, where the preposition predicate on takes the two arguments the picture and the wall; one of these semantic dependencies points up the syntactic hierarchy, whereas the other points down it. Finally, the predicate to help in takes the one argument Jim but is not directly connected to Jim in the syntactic hierarchy, which means that semantic dependency is entirely independent of the syntactic dependencies.