Each of these sentences consists of a subject and a predicate. But in each sentence different syntactic types of words or combinations of words constitute subject and predicate. Different combinations of parts of speech fulfilling the same syntactic function are said to be syntactically equivalent. It is possible to write rules describing syntactic equivalence. These rules are called phrase structure rules. These rules use special symbols designed exclusively for syntactic descriptions. Grammatical terms or graphic notation devices devised to describe language structure are examples of meta-language, defined roughly as language about language. The syntactic metalanguage used in writing phrase structure rules involves mainly abbreviations from English words for parts of speech.