What we have described as free morphemes fall into two categories. The first category is that set of ordinary nouns, adjectives and verbs that we think of as the words that carry the “content” of the messages we convey. These free morphemes are called lexical morphemes and some examples are: girl, man, house, tiger, sad, long, yellow, sincere, open, look, follow, break. We can add new lexical morphemes to the language rather easily, so they are treated as an “open” class of words.

Other types of free morphemes are called functional morphemes. Examples are and, but, when, because, on, near, above, in, the, that, it, them. This set consists largely of the functional words in the language such as conjunctions, prepositions, articles and pronouns. Because we almost never add new functional morphemes to the language, they are described as a “closed” class of words.