Formal means for describing the syntax of code switching are proposed and illustrated with examples from Puerto Rican Spanish and English. The role of code switching constraints in determining the way two monolingual grammars may be combined in generating discourse containing code switches is analyzed. Intrasentential code switching is characterized as a development requiring competence in the two component codes and skill in manipulating the codes concurrently. Based on code switches in recordings of 20 Puerto Rican bilingual or Spanish-dominant speakers, the distinction between surface and deep code switches, the free union of two grammars, a code switching grammar, superscript conventions, probabilistic grammars, code switching frequencies and rates, and rule frequencies are discussed. Two linguistic constraints of code switching are identified: the free morpheme constraint and the equivalence constraint. The performance data provided quantitative confirmation of the validity of these constraints. (RW)