Types of morphemes: Bound morphemes like a- appear only together with other morphemes to form a lexeme. Free morphemes like town can appear with other lexemes (town-hall) and alone. Allomorphs are variants of a morpheme, e.g. the plural marker in English is sometimes realized as /-z/, /-s/ or /-Iz/.
- Andrew Spencer, Morphological Theory, Blackwell, Oxford 1992