An allophonic rule is a phonological rule

Phonological rule

A phonological rule is a formal way of expressing a systematic phonological or morphophonological process or diachronic sound change in language. Phonological rules are commonly used in generative phonology as a notation to capture sound-related operations and computations the human brain performs...

 that indicates which allophone

Allophone

In phonology, an allophone is one of a set of multiple possible spoken sounds used to pronounce a single phoneme. For example, and are allophones for the phoneme in the English language...

 realizes a phoneme

Phoneme

In a language or dialect, a phoneme is the smallest segmental unit of sound employed to form meaningful contrasts between utterances....

 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

Phonetic transcription

Phonetic transcription is the visual representation of speech sounds . The most common type of phonetic transcription uses a phonetic alphabet, e.g., the International Phonetic Alphabet....

. Every language

Language

Language may refer either to the specifically human capacity for acquiring and using complex systems of communication, or to a specific instance of such a system of complex communication...

 has a set of allophonic rules.

In American English

American English

American English is a set of dialects of the English language used mostly in the United States. Approximately two-thirds of the world's native speakers of English live in the United States....

, the voiceless alveolar stop phoneme /t/ is realized as the alveolar flap allophone [ɾ] when it is preceded by a sonorant

Sonorant

In phonetics and phonology, a sonorant is a speech sound that is produced without turbulent airflow in the vocal tract; fricatives and plosives are not sonorants. Vowels are sonorants, as are consonants like and . Other consonants, like or , restrict the airflow enough to cause turbulence, and...

 phoneme other than an alveolar

Alveolar consonant

Alveolar consonants are articulated with the tongue against or close to the superior alveolar ridge, which is called that because it contains the alveoli of the superior teeth...

 nasal

Nasal consonant

A nasal consonant is a type of consonant produced with a lowered velum in the mouth, allowing air to escape freely through the nose. Examples of nasal consonants in English are and , in words such as nose and mouth.- Definition :...

 or lateral

Lateral consonant

A lateral is an el-like consonant, in which airstream proceeds along the sides of the tongue, but is blocked by the tongue from going through the middle of the mouth....

, and, at the same time, followed by an unstress

Stress (linguistics)

In linguistics, stress is the relative emphasis that may be given to certain syllables in a word, or to certain words in a phrase or sentence. The term is also used for similar patterns of phonetic prominence inside syllables. The word accent is sometimes also used with this sense.The stress placed...

ed vowel

Vowel

In phonetics, a vowel is a sound in spoken language, such as English ah! or oh! , pronounced with an open vocal tract so that there is no build-up of air pressure at any point above the glottis. This contrasts with consonants, such as English sh! , where there is a constriction or closure at some...

phoneme.

/t/ → [ɾ] | /+son -lat/ ＿ /+vwl -str/