The Horizontal and Vertical Dimension of Sentences

Another way of conveying information about the structure of a sentence is by means of so-called phrase-structure rules or rewrite-rules. It is important to note that our use of the notion ‘rule’ is different from that used in school grammar. In syntax, a rule is a direction for forming a sentence or part of a sentence. Such rules specify which combinations of words or morphemes form grammatical or well-formed sentences, and they represent the structural relations among members of a sentence. Typically, phrase-structure or rewrite-rules have the following form:

			A →	B + C

and can be paraphrased as “replace or rewrite the symbol A by the symbol B and the symbol C”. A simple example is the rule

			S →	NP + VP

which is understood as “replace the symbol S (for sentence) by the symbols NP (for noun phrase) and VP (for verb phrase)”. In this rule, the order of constituents is made explicit: the leftmost unit is first and the rightmost one is last. Phrase-structure or rewrite-rules can also be translated into tree diagrams, as shown in the following: