<P> In transformational grammar, systems of phrase structure rules are supplemented by transformation rules, which act on an existing syntactic structure to produce a new one (performing such operations as negation, passivization, etc .). These transformations are not strictly required for generation, as the sentences they produce could be generated by a suitably expanded system of phrase structure rules alone, but transformations provide greater economy and enable significant relations between sentences to be reflected in the grammar . </P> <P> An important aspect of phrase structure rules is that they view sentence structure from the top down . The category on the left of the arrow is a greater constituent and the immediate constituents to the right of the arrow are lesser constituents . Constituents are successively broken down into their parts as one moves down a list of phrase structure rules for a given sentence . This top - down view of sentence structure stands in contrast to much work done in modern theoretical syntax . In Minimalism for instance, sentence structure is generated from the bottom up . The operation Merge merges smaller constituents to create greater constituents until the greatest constituent (i.e. the sentence) is reached . In this regard, theoretical syntax abandoned phrase structure rules long ago, although their importance for computational linguistics seems to remain intact . </P> <P> Phrase structure rules as they are commonly employed result in a view of sentence structure that is constituency - based . Thus, grammars that employ phrase structure rules are constituency grammars (= phrase structure grammars), as opposed to dependency grammars, which view sentence structure as dependency - based . What this means is that for phrase structure rules to be applicable at all, one has to pursue a constituency - based understanding of sentence structure . The constituency relation is a one - to - one - or - more correspondence . For every word in a sentence, there is at least one node in the syntactic structure that corresponds to that word . The dependency relation, in contrast, is a one - to - one relation; for every word in the sentence, there is exactly one node in the syntactic structure that corresponds to that word . The distinction is illustrated with the following trees: </P> <P> The constituency tree on the left could be generated by phrase structure rules . The sentence S is broken down into smaller and smaller constituent parts . The dependency tree on the right could not, in contrast, be generated by phrase structure rules (at least not as they are commonly interpreted). </P>

Discuss phrase rank in relation to the structure of a modern english phrase