Rule. The s at the end of a pada is replaced by ru. So is the final ṣ of a pada that ends in the word sajuṣ. ru is the sound r carrying the marker u; the marker is an it, also called an anubandha, and is never pronounced. What the marker is for comes out at SK 166.
Example.
śivas (the word śiva with its nominative-singular ending) → its final s stands at the end of a pada → s is replaced by ru → śivar(u), which we write śivar.
sajuṣ (nominative singular) → the pada ends in the word sajuṣ, so its final ṣ → ru → sajur(u); the finished nominative singular is sajūḥ.
sajuṣau, sajuṣaḥ → the ṣ is not at the end of a pada → no ru; the ṣ stays.
parama-sajūḥ → the pada ends in the word sajuṣ → ru applies here too, by the tadanta reading.
Applies when / blocked by. The rule sits under the adhikāra padasya, "of a pada", and sa in the sūtra means the sound s (the a is only for pronunciation). So the rule reads "of the pada ending in s, and of the pada ending in sajuṣ"; the substitute then lands on the last sound only. The rule is an apavāda to jaśtva. Without that status, śivas would come out as śivad: jaśtva is the earlier Tripādī rule, so by SK 12 the later ru-rule ought to be invisible to it. But the ru-rule has no case of its own outside jaśtva's territory — every pada-final s is a jhal that jaśtva would take. A rule with no scope of its own (niravakāśa) must be the apavāda of the rule that covers all its cases, and an apavāda is not treated as invisible; it applies by the sheer authority of its being stated. Bhaṭṭoji's commentators cite the Bhāṣya for this.
Watch out. The ru is not yet a sound you can hear. It is a marked r, and every rule in this chapter that says roḥ ("of the ru") is looking for this marked r and nothing else.
For the s at the end of a pada, and for the word sajuṣ, ru is [substituted]. [This is] an apavāda (exception) to jaśtva.
sasajuṣo ruḥ — The word-division (cheda) is "sasajuṣaḥ ru"; the [visarga] r is [explained by] elision of the r by "ro ri." The analysis (vigraha) is: "sa and sajuṣ = sasajuṣau [dvandva dual], of those two (tayoḥ)." In the operation ru, the u is an it-marker (anubandha); its result (fruit) will be stated just below. By "sa" the sound s (sakāra) is intended; the a [in "sa"] is for the sake of pronunciation. The governing adhikāra "padasya" is qualified by "s" and by the word "sajuṣ"; hence the tadanta-vidhi [applies]. Meaning: the pada that ends in s and [the pada] that ends in the word sajuṣ — for that, ru is [substituted]; and by "alo 'ntyasya" it [applies] to the final sound. He states the upshot with "for the s at the end of a pada."
On "and for the word sajuṣ": the meaning is, of the ṣ at the end of the pada that ends in the word sajuṣ. And so, in "sajuṣau, sajuṣaḥ," there is no ru-substitution for the ṣ, because it is not at the end of a pada. By the tadanta-vidhi "the pada that ends in the word sajuṣ," there is no under-application (avyāpti) in "parama-sajūḥ"; nor should under-application be suspected in "sajūḥ" [alone], since by vyapadeśivadbhāva it is [treated as] ending in that [word sajuṣ]. The two paribhāṣās (paribhāṣā) — "vyapadeśivadbhāvo 'prātipadikena" and "iti-grahaṇavatā prātipadikena tadantavidhir na" — have as their sphere [the maxim] "pratyayagrahaṇe yasmāt...", not the sphere of "yena vidhis tadantasya"; this is clear in the Bhāṣya on the sūtra "asamāse niṣkādibhyaḥ."
Objection: in "śivas," the s would become d by jaśtva [via "jhalāṃ jaśo 'nte"], since ru-substitution, though para (later) relative to jaśtva, is asiddha (regarded as not having taken effect) with respect to it. To this he replies, "[it is] an apavāda to jaśtva": and so, since ru-substitution has no [other] scope (niravakāśa), there is no asiddhatva — this is the idea. This is stated in the Bhāṣya: "in [the domain of] pūrvatrāsiddham there is no vipratiṣedha, because of the non-existence of the later [rule's having taken effect]," and "an apavāda [prevails] by the authority of [its explicit] statement (vacana-prāmāṇya)."
- textual The digital source duplicates one sentence ('sajuṣ-śabdāntaṃ yat padaṃ tadantasya ṣakārasya... pravṛtti') and garbles śivas as śivasi; the translation renders each substantive statement once, following the evident readings. Stray Vedic-accent marks serve as quotation delimiters.
- commentary sk.162.t1 opens with TB on the head of the svādi-sandhi (this part was re-filed 2026-10-05 from the end of sk.161.t1): asked how ru can arise in śiva + arcya when no s is to be seen, the author points to svaujasamauṭ…: the su is there in the stem's own derivation, so ru follows; there is no visarga, since neither khar nor a pause follows, and by the akṛtavyūha paribhāṣā a pause yet to come does not count before arcya is joined. That lead-in is not in our kaumudī for this unit; the form śivo 'rcyaḥ is completed at SK 165. Recorded.
- commentary TB on the sūtra: padasya, carried forward, is qualified by s and sajuṣ, so the tadanta-vidhi applies; 'grahaṇavatā prātipadikena tadantavidhir na' does not block it for sajuṣ, because that paribhāṣā concerns affix rules. The pada ending in s or in sajuṣ takes ru, and by alo 'ntyasya only its final. On 'sajuṣśabdasya', TB, like BM, reads 'of the pada ending in it', so there is no over-extension to sajuṣau, sajuṣaḥ. Followed in k1 ('the s at the end of a pada', 'for the word sajuṣ') and in sutra_en's brackets.
- commentary TB, like BM, limits both paribhāṣās (vyapadeśivadbhāvo 'prātipadikena; grahaṇavatā…) to affix rules, so sajūḥ counts as ending in sajuṣ; but TB cites Haradatta on diva ut where BM cites the Bhāṣya on asamāse niṣkādibhyaḥ. TB adds that the Manoramā's 'by Kaiyaṭa and Haradatta' is faulted by the moderns, since Kaiyaṭa says nothing there, and rejects as forced a defence that reads 'diva ut' as covering indre ca. Recorded (§16); BM is translated as it stands.
- textual sk.162.t1 runs on past this sūtra: its last part ('yatvasyāsiddhatvād…', lemmas 'yatvasyāpavāda iti' and 'utvavidheḥ sāmarthyād iti') is TB on ato ror aplutād aplute (SK 163), whose kaumudī has those words; SK 163 has no TB block of its own. Not covered here.