Sandhi (Sanskrit: संधि, "joining") is a cover term for a wide variety of phonological processes that occur at morpheme or word boundaries. (Thus belonging to what is called morphophonology.) Examples include the fusion of sounds across word boundaries and the alteration of sounds due to neighboring sounds or due to the grammatical function of adjacent words. It occurs particularly prominently in Sanskrit phonology, hence its naming with a word from that language, but most languages have it.
While it may be extremely common in speech, it is typically ignored in spelling, as is the case in Finnish and, with the exception of the distinction between "a" and "an", English (see, for example, Linking R). External sandhi effects can sometimes become morphologized (i.e. apply only in certain morphological and syntactic environments) and, over time, turn into consonant mutations.
Most tonal languages have tone sandhi, in which the tones of words alter in complicated ways. For example: Mandarin has four tones: a high monotone, a rising tone, a falling-rising tone, and a falling tone. In the common greeting nǐ hǎo, both words would normally have the falling rising tone. However, this is difficult to say, so the tone on nǐ mutates into ní, although by orthographical rules the tone as written in Hanyu Pinyin does not change.
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