:Phonological rule:

Pronunciation GB: ˌfɒnəˈlɒdʒɪkl, GA:ˌfɑnəˈlɑdʒɪkl

A statement of the formː

A → B / X __ Y.

A B X and Y all represent segments, or feature specifications or, in the case of X Y, boundaries of various types. Additionally, A or B may represent 0 (=absence of segment). A is the input to the rule. B is the output. The sequence following the symbol / is the specification of the environment in which the rule operates. This may be absent, in which case the rule is said to be context-free. Either X or Y may be absent from the environment statement. An environment of the form X __ Y means between X and Y. One of the form X__ means following X and one of the form __Y means preceding Y. The arrow in a phonological rule is to be interpreted as meaning “is replaced by” or is “further specified as”. Rules of this format have been used to capture the relationship between child and adult pronunciation, the relationship between different accents of a language, the change of phonological forms over time and the relationship between underlying phonological forms and surface phonetic forms. Phonological theories which employ such rules usually specify that they are ordered in sequence, so that the output of an earlier rule becomes the input to the next rule in the sequence. Rule-based phonology has been replaced in recent years by theories which place emphasis on a more complex structure of phonological forms and the idea of phonological constraints.

See also Greek letter variable and braces.