164. The euphonic changes

Combinations of final स् s and र् r.

The euphonic changes of स् s and र् r are best considered together, because of the practical relation of the two sounds, in composition and sentence-collocation, as corresponding surd and sonant: in a host of cases स् s becomes र् r in situations requiring or favoring the occurrence of a sonant; and, much less often, र् r becomes स् s where a surd is required.

a. In internal combination, the two are far less exchangeable with one another: and this class of cases may best be taken up first.