At several points in the phrase structure rules examined in chapter 11, the symbol S for 'sentence' appeared inside another S or inside some constituent of another S. Thus, S may contain S, that is, sentences can be combined to form longer ones. This property is universal, although the structuring of combined sentences depends on the grammar of each language to some extent. Many languages, including Spanish and English, show two distinct types of sentence combinations, compound and complex.
A COMPOUND SENTENCE consists of two or more internal sentences (CLAUSES) joined by coordinating conjunctions (v. 11.1.1). It results from the application of the rule S → S (Conj S)n, that is, 'S may consist of S combined with the sequence Conj + S any number of times'. An example is "Tú saliste" y "la camarera te vio", whose coordinated or compound S Conj S structure is shown by phrase marker (a) in figure 13.1. (Note the introduction of two common conventions here: brackets are used to enclose and highlight constituents, clauses in this case; and in the trees of figure 13.1 the triangles abbreviate irrelevant internal details of constituents.)
A COMPLEX SENTENCE also contains two or more clauses, but instead of being loosely or additively joined as in compound S, one is built into the other. The larger S is called the MAIN or MATRIX CLAUSE; the internal one is described as EMBEDDED IN, SUBORDINATED TO, or a COMPLEMENT OF some constituent in the main clause. Whereas a coordinated clause plays no grammatical role in its compound mate, a subordinated clause carries out a syntactic function within the main clause, generally acting like a noun, adjective, or adverbial.
Some embedded clauses function like NP, as the Subj or Obj in the main clause. These are NOUN CLAUSES and are generated by the PSR NP → S, that is, 'an NP can consist of S alone'. This rule specifies the structure in tree (b) of figure 13.1 for Yo sé "que la camarera te vio". Note that the bracketed material in this example is itself a sentence (la camarera = NP, te vio = VP, and NP + VP = S), and that it carries out the same Obj function in the main clause as the final NP of Yo sé los