Phrase structure rules are ‘formulae’ that describe a given language’s syntax. Phrase structure rules break a natural language sentence down into its constituent parts (also known as syntactic categories. Phrase structure rules are usually of the form:
meaning that the constituent A is separated into the two sub-constituents B and C
This means that a sentence ‘S’ is separated into the two sub-constituents NP and VP in the order shown. Sub-constituents that are optional within a constituent is indicated in brackets:
This means that an ‘NP’ may be separated into the sub-constituents DET, AP and N; and that they would appear in that order if they do.