170. a. The स् s, as already noticed (145), becomes visarga before a pause.
b. It is retained unchanged only when followed by त् t or थ् th, the surd mutes of its own class.
c. Before the palatal and lingual surd mutes — च् c and छ् ch, ट् ṭ and ठ् ṭh — it is assimilated, becoming the sibilant of either class respectively, namely श् ç or ष् ṣ.
d. Before the guttural and labial surd mutes — क् k and ख् kh, प् p and फ् ph — it is also theoretically assimilated, becoming respectively the jihvāmūlīya and upadhmānīya spirants (69); but in practice these breathings are unknown, and the conversion is to visarga.
Examples are: to b. tatas te, cakṣus te; to c. tataç ca, tasyāç chāyā; pādaṣ ṭalati; to d. nalaḥ kāmam, puruṣaḥ khanati; yaçaḥ prāpa, vṛkṣaḥ phalavān.
171. The first three of these rules are almost universal; to the last one there are numerous exceptions, the sibilant being retained (or, by 180, converted into ṣ), especially in compounds; but also, in the Veda, even in sentence combination.
a. In the Veda, the retention of the sibilant in compounds is the general rule, the exceptions to which are detailed in the Vedic grammars.
b. In the later language, the retention is mainly determined by the intimacy or the antiquity and frequency of the combination. Thus, the final sibilant of a preposition or a word filling the office of a preposition before a verbal root is wont to be preserved; and that of a stem before a derivative of √kṛ, before pati, before kalpa and kāma, and so on. Examples are namaskāra, vācaspati, āyuṣkāma, payaskalpa.
c. The Vedic retention of the sibilant in sentence-collocation is detailed in full in the Prātiçākhyas. The chief classes of cases are: 1. the final of a preposition or its like before a verbal form; 2. of a genitive before a governing noun: as divás putráḥ, iḍás padé; 3. of an ablative before pári: as himávatas pári; 4. of other less classifiable cases: as dyāúṣ pitā́, tríṣ pūtvā́, yás pátiḥ, paridhíṣ pátāti, etc.
172. Before an initial sibilant — श् ç, ष् ṣ, स् s — स् s is either assimilated, becoming the same sibilant, or it is changed into visarga.
a. The native grammarians are in some measure at variance (see APr. ii. 40, note) as to which of these changes should be made, and in
