Rule. stu is a short name for s together with the dental class tu (t th d dh n; tu names the whole class by SK 14 of chapter 1). ścu is ś together with the palatal class cu (c ch j jh ñ). When a stu sound is in contact with a ścu sound, either before or after it, the stu sound is replaced by the corresponding ścu sound: s by ś, t by c, d by j, n by ñ, and so on.
Example.
hariḥ + śete → the final s (which stands for the visarga before this ś) is in contact with ś → s → ś → hariś śete.
rāmaḥ + cinoti → s in contact with c → s → ś → rāmaś cinoti.
sat + cit → the pada-final t first becomes d by the voicing rule SK 84 → d is in contact with c → d → j by this rule → j before the voiceless c becomes c by SK 121 (Section C) → sac cit.
śārṅgin + jaya → n in contact with j → n → ñ → śārṅgiñ jaya.
Applies when / blocked by. The change runs in both directions: the palatal may follow the dental (sat cit) or precede it (the case SK 112 goes on to limit). It is limited by SK 112 and, for the retroflex case, matched by SK 113.
Watch out. The rule replaces the dental, never the palatal. In rāmaś cinoti the c is untouched; it is the s that has moved.
In place of s and the dental class (ta-varga), when they are in conjunction with ś and the palatal class (ca-varga), ś and the palatal class come. hariś śete. rāmaś cinoti. sac-cit. śārṅgiñ-jaya.
stoḥ ścunā ścuḥ — stoḥ ścunā ścuḥ. "stoḥ": "s and tu" is a samāhāra-dvandva (aggregative compound) [so *stu* = the collection of s and the ta-varga]; the masculine gender [of the genitive form *stoḥ*] is an ārṣa [irregularity licensed to the sage]. Or it is an itaretarayoga-dvandva; if so, the singular number is an ārṣa. It is the same for "ścunā ścuḥ" too [i.e. *ścu* = ś + the ca-varga, by the same compound-analysis].
"ścunā": the instrumental is in the sense of "together with" (saha); "yoge" (in conjunction) is to be supplied by adhyāhāra. And so, even though there is no accompanying word *saha*, since that sense is understood, the instrumental is justified — precisely from this [mode of] statement [in the sūtra]. He states this — "of s and the dental class" etc.
Here the sthānins (substituends) and the ādeśas (substitutes) pair off respectively (yathāsaṅkhya): in place of s comes ś, and in place of the ta-varga comes the ca-varga. There too [within the ta-varga → ca-varga mapping] the order t-th-d-dh-n [→ c-ch-j-jh-ñ] is established from beginningless popular usage. In the next sūtra it will be stated that in "ścunā [yoge]" [the correspondence] is not yathāsaṅkhya; and so s and the ta-varga, in conjunction with ś and the ca-varga as the case may be (yathāsambhava), undergo ścutva.
"rāmaś śete": when *rāmas-śete* is present, from conjunction with ś, s becomes ś. To show the purpose of "in 'ścunā [yoge]' there is no yathāsaṅkhya," he illustrates s in conjunction with c as well — "rāmaś cinoti": when *rāmas-cinoti* is present, from conjunction with the ca-varga [here c], s becomes ś [not c]. For if yathāsaṅkhya were resorted to on the "ścunā yoge" [conditioning] side as well, then here, since s is not in conjunction with ś [but with c], ś would [wrongly] not result — this is the idea.
"sac-cit": when *sat-cit* is present — because ścutva is asiddha [not yet effective], by jaśtva the t becomes d (sat → sad); then by ścutva that becomes j (sad → saj, before c); then by cartva ("khari ca," A 8.4.55) that becomes c (saj → sac); such is the form.
"śārṅgiñ-jaya": when *śārṅgin-jaya* is present, from conjunction with the ca-varga [j], by ścutva n is replaced by ñ (śārṅgin → śārṅgiñ).
- textual The digital source shows minor OCR artifacts (e.g. 'शाङ्ग-' for śārṅga-, 'शार्ङ्गिञ्जय' in the Kaumudī vs 'शाङ्गन्' in the Bālamanoramā for the same word; a stray 'ऽ' rendered as '।ञ'). The evidently intended readings are followed; nothing is silently emended in the source layer. The example word is śārṅgiñ-jaya throughout.