169. As the final consonant of derivative stems

As the final consonant of derivative stems and of inflected forms, both of declension and of conjugation, s is extremely frequent; and its changes form a subject of first-rate importance in Sanskrit euphony. The r, on the other hand, is quite rare.

a. The r is found as original final in certain case-forms of stems in ṛ or ar (369 ff.); in root-stems in ir and ur from roots in ṛ (383 b); in a small number of other stems, as svàr, áhar and ū́dhar (beside áhan and ū́dhan: 430), dvā́r or dur, and the Vedic vádhar, uṣarvasar-, vanar-, çrutar-, sapar-, sabar-, athar- (cf. 176 c); in a few particles, as antár, prātár, púnar; and in the numeral catúr (482 g).

b. The euphonic treatment of s and r yielding precisely the same result after all vowels except a and ā, there are certain forms with regard to which it is uncertain whether they end in s or r, and opinions differ respecting them. Such are ur (or us) of the gen.-abl. sing. of ṛ-stems (371 c), and us (or ur) of the 3d plur. of verbs (550 c).