In phonology, assimilation is a common phonological process by which one sound becomes more like a nearby sound. This can occur either within a word or between words. It occurs in normal speech, and it becomes more common in more rapid speech. In rapid speech, for example, "handbag" is often pronounced /ˈhæmbæɡ/. The pronunciations /ˈhænˌbæɡ/ or /ˈhændˌbæɡ/ are however common in normal speech whereas the word "cupboard", for example, is always pronounced /ˈkʌbərd/, never /ˈkʌpˌbɔrd/, and even in slow, highly articulated speech.
As in these examples, sound segments typically assimilate to a following sound (this is called regressive or anticipatory assimilation), but they may also assimilate to a preceding one (progressive assimilation). While assimilation most commonly occurs between immediately adjacent sounds, it may occur between sounds separated by others ("assimilation at a distance").
A related process is coarticulation, where one segment influences another to produce an allophonic variation, such as vowels acquiring the feature nasal before nasal consonants when the velum opens prematurely or /b/ becoming labialised as in "boot". This article describes both processes under the term assimilation.
The physiological or psychological mechanisms of coarticulation are unknown; 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). Assimilations to an adjacent segment are vastly more frequent than assimilations to a non-adjacent one. These radical asymmetries might contain hints about the mechanisms involved, but they are unobvious.
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". Many find these 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 one another 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.
Anticipatory assimilation to an adjacent segment 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 assimilate to a following /t/:
Anticipatory assimilation at a distance is rare, and usually merely an accident in the history of a specific word.
However, the diverse and common assimilations known as umlaut, wherein the phonetics of a vowel are influenced by the phonetics of a vowel in a following syllable, are both commonplace and in the nature of sound laws. Such changes abound in the histories of Germanic languages, Romance, Insular Celtic, Albanian, and many others.
Examples: in the history of English, a back vowel becomes front if a high front vowel or semivowel (*i, ī, j) is in the following syllable, and a front vowel becomes higher, if it is not already high:
Contrariwise, Proto-Germanic *i and *u > e, o respectively before *a in the following syllable (Germanic a-mutation), although this had already happened significantly earlier:
Another example of a regular change is the sibilant assimilation of Sanskrit, wherein if there were two different sibilants as the onset of successive syllables, a plain /s/ was always replaced by the palatal /ɕ/:
Lag assimilation to an adjacent segment is tolerably common, and often has the nature of a sound law.
Proto-Indo-European *-ln- becomes -ll- in both Germanic and Italic. Thus *ḱl̥nis "hill" > PreLat. *kolnis > Lat. collis; > PGmc *hulniz, *hulliz > OE hyll /hyl/ > hill. The enclitic form of English is, eliding the vowel, becomes voiceless when adjacent to a word-final voiceless non-sibilant. Thus it is [ɪtɪz], that is [ðætɪz] > it's [ɪts], that's [ðæts].
For many if not most speakers of Polish, /v/ becomes /f/ after a voiceless obstruent:
Lag assimilation at a distance is rare, and usually sporadic (except when part of something bigger, as in the Skt. śaśa- example, above): Greek leirion > Lat. līlium "lily".
In vowel harmony, a vowel's phonetics is often influenced by that of a preceding vowel. Thus for example most Finnish case markers come in two flavors, with /ɑ/ (written a) and /æ/ (written ä) depending on whether the preceding vowel is back or front. However, it is a difficult to know where and how in the history of Finnish an actual assimilatory change took place. The distribution of pairs of endings in Finnish is just that, and is not in any sense the operation of an assimilatory innovation (though probably the outbirth of such an innovation in the past).
Proto-Italic *dw > Latin b, as in *dwis "twice" > Lat. bis. Also, Old Latin duellum > Latin bellum "war".
Proto-Celtic *sw shows up in Old Irish in initial position as s, thus *swesōr "sister" > OIr siur */ʃuɾ/, *spenyo- > *swinea- > *swine "nipple" > sine. However, when preceded by a vowel, the *sw sequence becomes /f/: má fiur "my sister", bó tri-fne "a cow with three teats". There is also the famous change in P-Celtic of *kʷ -> p. Proto-Celtic also underwent the change *gʷ -> b.