Assimilation is a common phonological process in which a sound becomes more similar to a nearby sound. Assimilation can be classified into two types, progressive and regressive. Let's look at a few examples of assimilation in English:
The word ten is pronounced with an alveolar nasal. However, in the word tenth the nasal is dental. The reason is that it is followed by an interdental fricative. The nasal becomes more similar to the following sound. This is an example of regressive assimilation.
The words kiss and cool are both pronounced with a velar plosive. However, they aren't identical. The /k/ in kiss has an advanced articulation. This is because the vowel in kiss is a front vowel. In cool the vowel is back. The front vowel of kiss triggers the advanced articulation of the plosive. This is an example of regressive assimilation.
The words happy and home have the same initial consonant, but they're pronounced a bit differently. The glottal fricative of home is produced with lip rounding. As a result, the /h/ is labialized. This isn't the case in happy. The /h/ of home is labialized because it is followed by a rounded vowel. This is an example of regressive assimilation.
In the words hand and hat, the vowel is the same. However, the vowel of hand is nasalized. The reason is that it's followed by a nasal in the same syllable. The nasal triggers nasalization of the vowel. This is also an example of regressive assimilation.
However, assimilation can also be progressive. The phrase Let's go has a word-final consonant in let's and a word-initial consonant in go. When the two words are articulated quickly, the /g/ of go becomes a /k/. This is a voicing asssimilation. The voiceless alveolar fricative of let's devoices the velar plosive of go. This is an example of progressive assimilation.
English has many examples of assimilation, a phonological process which causes one speech segment to become more like another. Assimilation can happen within a word or even across word boundaries. Assimilation can be further divided into two types, regressive and progressive. Of these two types, regressive is more common.