Grammar is the study of how meaningful elements (morphemes) within a language can be combined into utterances. Morphemes can either be free or bound. If they are free to be moved around within an utterance, they are usually called words, and if they are bound to other words or morphemes, they are called affixes. The way in which meaningful elements can be combined within a language is governed by rules. The rules obtaining for the internal structure of words are called morphology. The rules of the internal structure of the phrases and sentences are called syntax.[17]