Underlying representation in the context of "Optimality theory"

In historical linguistics, a sound change is a change in the pronunciation of a language. A sound change can involve the replacement of one speech sound (or, more generally, one phonetic feature value) by a different one (called phonetic change) or a more general change to the speech sounds that exist (phonological change), such as the merger of two sounds or the creation of a new sound. A sound change can eliminate the affected sound, or a new sound can be added. Sound changes can be environmentally conditioned if the change occurs in only some sound environments, and not others.

The term "sound change" refers to diachronic changes, which occur in a language's sound system. On the other hand, "alternation" refers to changes that happen synchronically (within the language of an individual speaker, depending on the neighbouring sounds) and do not change the language's underlying system (for example, the -s in the English plural can be pronounced differently depending on the preceding sound, as in bet[s], bed[z], which is a form of alternation, rather than sound change). Since "sound change" can refer to the historical introduction of an alternation (such as postvocalic /k/ in the Tuscan dialect, which was once [k] as in di [k]arlo 'of Carlo' but is now [h] di [h]arlo and alternates with [k] in other positions: con [k]arlo 'with Carlo'), that label is inherently imprecise and must often be clarified as referring to either phonemic change or restructuring.

A voiced velar nasal, also known as eng, engma, or agma (from Greek ἆγμα âgma 'fragment'), is a type of consonantal sound used in some spoken languages. It is the sound of ng in English sing as well as n before velar consonants as in English and ink. The symbol in the International Phonetic Alphabet that represents this sound is ⟨ŋ⟩. The IPA symbol ⟨ŋ⟩ is similar to ⟨ɳ⟩, the symbol for the retroflex nasal, which has a rightward-pointing hook extending from the bottom of the right stem, and to ⟨ɲ⟩, the symbol for the palatal nasal, which has a leftward-pointing hook extending from the bottom of the left stem.

While almost all languages have /m/ and /n/ as phonemes, /ŋ/ is rarer. Half of the 469 languages surveyed in Anderson (2008) had a velar nasal phoneme; as a further curiosity, many of them limit its occurrence to the syllable coda. The velar nasal does not occur in many of the languages of the Americas, the Middle East, or the Caucasus, but it is extremely common among Australian Aboriginal languages, languages of Sub-Saharan Africa, East Asian and Southeast Asian languages, and Polynesian languages. In many languages that do not have the velar nasal as a phoneme, such as most of the Romance languages, it occurs as an allophone of /n/ before velar consonants. This kind of assimilation can even be found in languages with phonemic voiced velar nasals, such as English. An example of this phenomenon is the word income; its underlying representation, /ˈɪnˌkʌm/, can be realized as either [ˈɪnˌkʌm] or [ˈɪŋˌkʌm].

A phonological rule is a formal way of expressing a systematic phonological or morphophonological process in linguistics. 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 sounds 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. When an underlying form has multiple surface forms, this is often referred to as allophony. For example, the English plural written -s may be pronounced as [s] (in "cats"), [z] (in "cabs", "peas"), or as [əz] (in "buses"); these forms are all theorized to be stored mentally as the same -s, but the surface pronunciations are derived through a series of phonological rules.