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Assimilation is a sound change in which some phonemes 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 rather than. In other cases, the change is accepted as canonical for that word or phrase, especially if it is recognized in standard spelling: implant pronounced with , composed historically of in + plant.
English "handbag" is often pronounced /ˈhæmbæɡ/ in rapid speech because the and sounds are both bilabial consonants, and their places of articulation are similar. However, the sequence - has different places but similar manner of articulation and is sometimes elided, which sometimes causes the canonical phoneme to assimilate to before the. 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 pronounced /ˈkʌbərd/, never */ˈkʌpbɔːrd/, even in slow, highly-articulated speech.