A term used in syntax and morphology to refer to the phonological modification of grammatical forms which have been juxtaposed. Sandhi forms are forms which have undergone specific modifications in specific circumstances (i.e. various sandhi rules have applied). Assimilation and dissimilation are two widespread tendencies which could be classified under this heading. In languages where sandhi forms are complex, a distinction is sometimes made between external sandhi (sandhi rules which operate across word boundaries) and internal sandhi (rules which operate within words).