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 when producing or comprehending spoken language. They may use phonetic notation or distinctive features or both.
John Goldsmith (1995) defines phonological rules as mappings between two different levels of sound representation—in this case, the abstract or underlying level and the surface level—and Bruce Hayes (2009) describes them as "generalizations" about the different ways a sound can be pronounced in different environments. That is to say, phonological rules describe how a speaker goes from the abstract representation stored in their brain, to the actual sound they articulate when they speak. In general, phonological rules start with the underlying representation of a sound (the phoneme that is stored in the speaker's mind) and yield the final surface form, or what the speaker actually pronounces. For example, the English plural -s may be pronounced as [s] (in "cats"), [z] (in "cabs"), or as [əz] (in "buses"); these forms are all stored mentally as the same -s, but the surface pronunciations are derived through a phonological rule.
In most dialects of American English, speakers have a process known as intervocalic alveolar flapping that changes the consonants /t/ and /d/ into a quick flap consonant ([ɾ] in words such as "butter" ([ˈbʌɾɹ]) and "notable" ([ˈnoʊɾəbl]).[note 1] The stop consonants /t/ and /d/ only become a flap in between two vowels, where the first vowel is stressed and the second is stressless. It is common to represent phonological rules using formal rewrite rules in the most general way possible. Thus, the intervocalic alveolar flapping described above can be formalized as
Format and notation 
The rule given above for intervocalic alveolar flapping describes what sound is changed, what the sound changes to, and where the change happens (in other words, what the environment is that triggers the change). The illustration below presents the same rule, with each of its parts labelled and described.
Taken together and read from left to right, this notation of the rule for intervocalic alveolar flapping states that any alveolar stop consonant (/t/ or /d/) becomes a tap ([ɾ]) in the environment where it is preceded by a stressed vowel and followed by an unstressed one.
Phonological rules are often written using distinctive features, which are (supposedly[note 3]) natural characteristics that describe the acoustic and articulatory makeup of a sound; by selecting a particular bundle, or "matrix," of features, it is possible to represent a group of sounds that form a natural class and pattern together in phonological rules. For example, in the rule above, rather than writing /t/ and /d/ separately, phonologists may write the features that they have in common, thus capturing the whole set of sounds that are stop consonants and are pronounced by placing the tongue against the alveolar ridge. In the most commonly used feature system, the features to represent these sounds would be [+delayed release, +anterior, -distributed], which describe the manner of articulation and the position and shape of the tongue when pronouncing these two sounds. But rules are not always written using features; in some cases, especially when the rule applies only to a single sound, rules are written using the symbols of the International Phonetic Alphabet.
Hayes (2009) lists the following characteristics that all phonological rules have in common:
- Language specificity: A phonological rule that is present in one language may not be present in other languages, or even in all dialects of a given language.
- Productivity: Phonological rules apply even to new words. For example, if an English speaker is asked to pronounce the plural of the nonsense word "wug" (i.e. "wugs"), they pronounce the final s as [z], not [s], even though they have never used the word before. (This kind of test is called the wug test.)
- Untaught and unconscious: Speakers apply these rules without being aware of it, and they acquire the rules early in life without any explicit teaching.
- Intuitive: The rules give speakers intuitions about what words are "well-formed" or "acceptable"; if a speaker hears a word that does not conform to the language's phonological rules, the word will sound foreign or ill-formed.
|Sound change and alternation|
Phonological rules can be roughly divided into four types:
- Assimilation: When a sound changes one of its features to be more similar to an adjacent sound. This is the kind of rule that occurs in the English plural rule described above—the -s becomes voiced or voiceless depending on whether or not the preceding consonant is voiced.
- Dissimilation: When a sound changes one of its features to become less similar to an adjacent sound, usually to make the two sounds more distinguishable. This type of rule is often seen among people speaking a language that is not their native language, where the sound contrasts may be difficult.
- Insertion: When an extra sound is added between two others. This also occurs in the English plural rule: when the plural morpheme -s is added to "bus," "bus-s" would be unpronouncable, so a short vowel (the schwa, [ə]) is inserted between the two [s]s.
- Deletion: When a sound, such as a stressless syllable or a weak consonant, is not pronounced; for example, most American English speakers do not pronounce the [d] in "handbag".
- See International Phonetic Alphabet for information about how to read these transcriptions.
- See Place of articulation and Manner of articulation for a description of these terms.
- Some phonologists have questioned the "naturalness" or "innateness" of distinctive features. See, for example, Mielke, Jeff (2005). "Ambivalence and ambiguity in laterals and nasals". Phonology 22 (2): 193. doi:10.1017/S0952675705000539.
- Goldsmith 1995:2.
- Hayes 2009:26.
- Idsardi, William James (2 September 2003). "LING 101: Phonology". University of Delaware. pp. A Rule of English. Retrieved 7 March 2009.[dead link]
- Idsardi, William James (2 September 2003). "LING 101: Phonology". University of Delaware. pp. The pronunciation of the English plural. Retrieved 7 March 2009.[dead link]
- Hayes 2009:28
- Hayes 2009:71.
- Hayes 2009:79; 84–85.
- Hayes 2009:92.
- Hayes 2009:26–7.
- Schramm, Andreas (17 March 2001). "Lesson 9.2: Phonological Rules". Hamline University. Retrieved 7 March 2009.