Mutation or Sandhi is the fusion of sounds across word boundaries, or alteration of sounds due to neighboring sounds, or because of the grammatical function of adjacent words. It is particularly prominent in Sanskrit phonology, hence called by a word from that language, but most languages have it. Internal sandhi is the alteration of sounds within words at morpheme boundaries, as in sympathy.

The alternation of English article "the" is also mutation, because saying /­ə Špl/ (the apple) is awkward, so we say /­iː Špl/ instead.

Mutation is extremely critical in all of the Celtic languages. There are three kinds in Welsh, all affecting the initial consonant of a word: Lenition or softening, in which voiceless consonants become voiced, and voiced stops become fricatives. Nasal, in which sounds become their corresponding nasal, thus, p>mh, b>m, g>ng etc. Spirant mutation is the final kind, in which k and g become the affricates. Breton has two mutations. Because of these mutations, it is extremely difficult to use a celtic language dictionary without knowledge of mutation patterns.

The Sindarin language created by J. R. R. Tolkien has lenition patterns based on those of Welsh. A word is softened if it: is the second part of a compound, anything after an article, or the subject of a verb and next to it. Thus, we get certh, rune, and i gerth, the rune.

Most tonal languages have tone sandhi, in which the tones of words words are altered 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 2ni 3hao, both words would normally have the falling rising tone. However, this is difficult to say, so the tone on 3ni mutates into 2ni.