A phonological rule is a method for describing the way in which individual sounds are produced in spoken language. These rules are written out in a specialized notation that codifies the way in which a sound or group of sounds is altered by appearing in a specific linguistic context. Phonological rules vary between languages and dialects, and they reflect the common pronunciation habits of various linguistic groups. By studying the way that a particular phonological rule operates in spoken language, linguists are able to determine the physiological and neurological mechanisms that translate mental language into spoken language.
A complete phonological rule includes the underlying sound that is altered, the environment in which it is altered and the specific alteration that takes place. Rules might deal with groups of underlying sounds if all the sounds undergo the same alteration when placed in the same linguistic environment. The linguistic environment describes the types of sounds that must exist before or after the underlying sound for the alteration to take place, and it can include both positive and negative features. For example, a phonological rule might describe an alteration that takes place after a consonant in a stressed syllable and before a vowel in a stressless syllable. The alteration is generally expressed in the International Phonetic Alphabet (IPA) or as a description of the common features between multiple altered sounds, and it communicates the sound that results from the altered underlying sounds.
Phonological rules are broadly divided into four main groups, which are distinguished by the type of alteration that takes place. Assimilation is the alteration of a sound that makes it more similar to neighboring sounds, making the word easier to pronounce by eliminating some movements of the speech organs. Dissimilation is an alteration which causes a sound to be less similar to neighboring sounds, which can make certain sounds more likely to be heard by the listener. Insertion is the introduction of an unwritten sound between highly similar or difficult to pronounce sounds, such as the "-e" commonly inserted when pluralizing a word ending with "s." Deletion or truncation occurs when a sound is masked or dropped entirely from a word.
Although they are called rules, phonological rules do not imply a correct or preferred mode of pronunciation. They can be, and often are, written to reflect nonstandard dialects and modes of speech. Phonological rules are straightforward descriptions of pronunciation. They generally are utilized to study the interaction between mental and physical speech and the physiology of the speech organs rather than the cultural aspects of a particular pronunciation.