Morphemes may have lexical meaning, as the word bird, or syntactic meaning, as the plural – s (see inflection; etymology). Words are minimal free forms, but a word may contain more than one morpheme. For example, treatment contains two, treat and the derivational noun-forming suffix -ment. In traditional grammar, parts of speech are defined semantically, i.e., a noun is a person, place, or thing; but in linguistic morphology, parts of speech are defined according to their syntactic function: The difference between nouns and verbs is that they cannot appear in the same environment in a sentence. One method of language classification is based on structure; languages are classified according to the degree of synthesis, or the number of morphemes per word. Analytic languages, such as Chinese, have only one morpheme per word, while in synthetic languages one word represents more than one morpheme; in the case of some Native American languages, a single word may have so many morphemes that it is the equivalent of an English sentence. The list of morphemes and their meanings (see semantics) in a language is usually not part of a grammar but is isolated in a dictionary or vocabulary.