A morpheme is the smallest meaningful constituent of a linguistic expression. The field of linguistic study dedicated to morphemes is called morphology. In English, morphemes are often but not necessarily words. Morphemes that stand alone are considered roots (such as the morpheme cat); other morphemes, called affixes, are found only in combination with other morphemes. For example, the -s in cats indicates the concept of plurality but is always bound to another concept to indicate a specific kind of plurality. These sample English words have the following morphological analyses: