Shana Poplack's model of code-switching is an influential theory of the grammar of code-switching. In this model, code-switching is subject to two constraints. The "free-morpheme constraint" stipulates that code-switching cannot occur between a lexical stem and bound morphemes. Essentially, this constraint distinguishes code-switching from borrowing. Generally, borrowing occurs in the lexicon, while code-switching occurs at either the syntax level or the utterance-construction level. The "equivalence constraint" predicts that switches occur only at points where the surface structures of the languages coincide, or between sentence elements that are normally ordered in the same way by each individual grammar. For example, the sentence: "I like you "porque eres simpático"" ("I like you "because you are nice"") is allowed because it obeys the syntactic rules of both Spanish and English. Cases like the noun phrases "the casa white" and "the blanca house" are ruled out because the combinations are ungrammatical in at least one of the languages involved. Spanish noun phrases are made up of determiners, then nouns, then adjectives, while the adjectives come before the nouns in English noun phrases. "The casa white" is ruled out by the equivalence constraint because it does not obey the syntactic rules of English, and "the blanca house" is ruled out because it does not follow the syntactic rules of Spanish.