Languages express concepts via two types of meaningful sound: grammatical morphemes and lexemes. Lexemes are simply noun, verb, and adjective stems which express general concepts. Grammatical morphemes are sometimes referred to as 'function words'. 'Grammatical morpheme' is a better term for the functions of language are expressed not only by words but by suffixes, prefixes, and unaccented particles, too. They belong to closed classes to which speakers may not add nor subtract and never refer to general cognitive categories. Grammatical morphemes are those bits of linguistic sound which mark the grammatical categories of language (Tense, Number, Gender, Aspect), each of which has one or more functions (Past, Present, Future are functions of Tense; Singular and Plural are functions of Number). So suffixes like -s and -ed as well as particles like the (Definite) and not (Negative) are grammatical morphemes because they express grammatical functions, not general concepts.