8.3.85
मातुःपितुर्भ्यामन्यतरस्याम्
Operational rule vidhi
ष् is optionally substituted for the स् of स्वसृ after मातुर् and पितुर् in a compound.
Word by word (padaccheda)
Left context मातुः- पितुर्भ्याम् pañcamī · dvivacanaRight context अन्यतरस्याम् saptamī · ekavacana
- Left context Pañcamī (5th case): "after X", the operation applies to what follows (1.1.67 तस्मादित्युत्तरस्य). Also used for limits and sources.
- Right context Saptamī (7th case): "when X follows", the operation applies to what precedes (1.1.66 तस्मिन्निति निर्दिष्टे पूर्वस्य). Also a condition or domain ("in the sense of").
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Full reading
मातुः-
Pāṇini leaves out words that carry over from earlier sūtras. Expand to see the rule as a reader actually understands it.
Commentary
ष् is optionally substituted for the स् of स्वसृ after मातुर् and पितुर् in a compound.
Thus मातुःस्वसा or मातुःष्वसा; पितुःस्वसा or पितुःष्वसा ॥ The word मातुर् and पितुर् end in र्, which is changed to visarga (See 8.2.24). In fact, the word पितुर् is so exhibited in the sutra itself, with a र्, the word मातुः therefore, by the rule of साहचर्य is also to be understood as a र् ending word.
Question:- Well, if this be so, when this र् is changed to a visarga, and the visarga changed to स् by 8.3.36, then there would be no ष change ?
Answer:- The ष change would take place both after the visarga-ending or स-ending words, on the maxim एकदेशविकृतस्यानन्यत्वात् "That which has undergone a change in regard to one of its parts, is by no means, in consequence of this change, something else than what it was before the change had taken place". Therefore, these words are taken to end in र् ॥ For if they were taken to end in स्, then the rule would not apply when they ended in a visarga. If they be supposed to end in a visarga, then the form पितुर्भ्याम् with a र् before भ्याम् in the sutra is hard to explain.
The word समासे is understood here also. Therefore not here मातुः स्वसा when the words are used separately in a sentence.
Śrīśa Chandra Vasu, The Ashtadhyayi of Panini (1897), via ashtadhyayi.com.