Syntactic development involves the ways that various morphemes are ordered or combined in sentences. Morphemes, which are basic units of meaning in language, get added as children learn to produce simple sentences and become more precise speakers. Morphemes can be whole words (like "happy") or parts of words that change meaning of words ("un"happy). Brown proposed a stage model that describes the various types of morphological structures that are developed in English and the age range within which they are normally acquired.