Assimilation is a sound change in which some phonemes (typically consonants or vowels) change to become more similar to other nearby sounds. A common type of phonological process across languages, assimilation can occur either within a word or between words.
It occurs in normal speech but becomes more common in more rapid speech. In some cases, assimilation causes the sound spoken to differ from the normal pronunciation in isolation, such as the prefix in- of English input pronounced with phonetic [m] rather than [n]. In this case, [n] becomes [m] since [m] is more phonetically similar to [p]. In other cases, the change is accepted as canonical for that word or phrase, especially if it is recognized in standard spelling: implosion pronounced with [m], composed of in- + -plosion (as in explosion).
English "handbag" (canonically /ˈhændbæɡ/) is often pronounced /ˈhæmbæɡ/ in rapid speech because the [m] and [b] sounds are both bilabial consonants, and their places of articulation are similar. However, the sequence [d]-[b] has different places but similar manner of articulation (voiced stop) and is sometimes elided, which sometimes causes the canonical [n] phoneme to assimilate to [m] before the [b]. The pronunciations /ˈhænbæɡ/ or /ˈhændbæɡ/ are, however, common in normal speech.
In contrast, the word "cupboard", although it is historically a compound of "cup" /kʌp/ and "board" /bɔːrd/, is always generally pronounced /ˈkʌbərd/, and almost never /ˈkʌpbɔːrd/.[note 1]
Like in those examples, sound segments typically assimilate to a following sound,[note 2] but they may also assimilate to a preceding one.[note 3] Assimilation most commonly occurs between immediately adjacent-sounds but may occur between sounds that are separated by others.[note 4]
Assimilation can be synchronic, an active process in a language at a given point in time, or diachronic, a historical sound change.
A related process is coarticulation in which one segment influences another to produce an allophonic variation, such as vowels becoming nasalized before nasal consonants (/n, m, ŋ/) when the soft palate (velum) opens prematurely or /b/ becoming labialized as in "boot" [bʷuːt̚] or "ball" [bʷɔːɫ] in some accents. This article describes both processes under the term assimilation.
The physiological or psychological mechanisms of coarticulation are unknown, and coarticulation is often loosely referred to as a segment being "triggered" by an assimilatory change in another segment. In assimilation, the phonological patterning of the language, discourse styles and accent are some of the factors contributing to changes observed.
There are four configurations found in assimilations:
Although all four occur, changes in regard to a following adjacent segment account for virtually all assimilatory changes and most of the regular ones.[citation needed] Assimilations to an adjacent segment are vastly more frequent than assimilations to a nonadjacent one. Those radical asymmetries might contain hints about the mechanisms involved, but they are not obvious.
If a sound changes with reference to a following segment, it is traditionally called "regressive assimilation". Changes with reference to a preceding segment are traditionally called "progressive".[1] Many[2] find those terms confusing, as they seem to mean the opposite of the intended meaning. Accordingly, a variety of alternative terms have arisen, not all of which avoid the problem of the traditional terms. Regressive assimilation is also known as right-to-left, leading, or anticipatory assimilation. Progressive assimilation is also known as left-to-right, perseveratory, preservative, lagging, or lag assimilation. The terms anticipatory and lag are used here.
Occasionally, two sounds (invariably adjacent) may influence each other in reciprocal assimilation. When such a change results in a single segment with some of the features of both components, it is known as coalescence or fusion.
Assimilation occurs in two different types: complete assimilation, in which the sound affected by assimilation becomes exactly the same as the sound causing assimilation, and partial assimilation, in which the sound becomes the same in one or more features but remains different in other features.
Tonal languages may exhibit tone assimilation (in effect tonal umlaut), but sign languages also exhibit assimilation when the characteristics of neighbouring cheremes may be mixed.
Anticipatory assimilation to an adjacent segment[3] is the most common type of assimilation by far, and typically has the character of a conditioned sound change, i.e., it applies to the whole lexicon or part of it. For example, in English, the place of articulation of nasals assimilates to that of a following stop (handkerchief is pronounced [hæŋkɚtʃif], handbag in rapid speech is pronounced [hæmbæɡ]).
In Italian, voiceless stops assimilated historically to a following /t/:
Italian otto, letto and sotto are examples of historical restructuring: otto and letto no longer contain /kt/ pronounced [tt], and sotto is no longer the structure /bt/ subject to the partial assimilation of devoicing of /b/ and full assimilation to produce [tt]. Over time, phonetic [tt] as a frequent assimilation of /kt/ and /bt/ was rather reinterpreted as reflecting /tt/. The structural sequence /kt/ is now all but absent in Italian, since all items in popular speech underwent the same restructuring, /kt/ > /tt/. On the rare occasion that Italian /kt/ is encountered, however, the same assimilation that triggered the restructuring can occur at the phonetic level. For example, the medical term ictus 'stroke', a relatively recent direct borrowing from Latin, is usually pronounced [ˈiktus] in deliberate speech, but [ˈittus] is frequent in more casual registers.
There has been a notable change recognized across a variety of English dialects regarding the pronunciation of the /tr/ and /dr/ consonant clusters. Starting around the mid-20th century,[4] the alveolar stop in /tr, dr/ has slowly been replaced by a post-alveolar affricate instead, resulting in the all-postalveolar consonant clusters [tʃɹ] and [dʒɹ].[5] This phenomenon also occurs in /str/, resulting in the all-postalveolar consonant cluster [ʃtʃɹ]. The affrication of /tr, dr/ has been seen in American English, British English, Australian English, and New Zealand English.[5] It is suspected that this change has occurred due to assimilation.[5]
One of the first papers that discussed the affrication of /tr, dr/ is "Pre-School Children's Knowledge of English Phonology" by Charles Read, published in 1971.[6] The study discussed in this paper focuses on how children in pre-school analyze the phonetic aspect of language in order to dete