Allophonic rule

An allophonic rule is a phonological rule that says which allophone realizes a phoneme in a given phonemic environment. In other words, an allophonic rule is a rule that converts the phonemes in a phonemic transcription into the allophones of the corresponding phonetic transcription. Every dialect has a set of allophonic rules.

For example, in General American, a non-regional dialect of American English, the voiceless alveolar stop phoneme is realized as the alveolar flap allophone  when it is preceded by a non-lateral sonorant phoneme and, at the same time, followed by an unstressed vowel phoneme: