171. First three of these rules

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 fitting 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.