in the analysis of word forms (morphology). Two different word forms (allomorphs) can actually be different "faces" of one and the same word (morpheme). An example is the English indefinite articles a and an. The usages an aardvark and a bear are grammatical, but the usages *a aardvark and *an bear are ungrammatical (as is marked with "*" in linguistics).

:The form an is used before a word that begins with a vowel