मातृपितृभ्यां स्वसा

Operational rule vidhi

ष् is optionally substituted for the स् of स्वसृ after मातृ and पितृ in a compound.

Word by word (padaccheda)

Left context मातृ- पितृभ्याम् pañcamī · dvivacanaSubstitute / term स्वसा prathamā · ekavacana

Left context Pañcamī (5th case): "after X", the operation applies to what follows (1.1.67 तस्मादित्युत्तरस्य). Also used for limits and sources.
Substitute / term Prathamā (1st case): in operational rules, the substitute or the thing introduced; in definitions, the term or its referent.

Dotted words are technical terms. Hover or tap one to see its card.

मातृ-पितृभ्याम् स्वसा

Pāṇini leaves out words that carry over from earlier sūtras. Expand to see the rule as a reader actually understands it.

ष् is optionally substituted for the स् of स्वसृ after मातृ and पितृ in a compound.
As मातृष्वसा, पितृष्वसा ॥ See 6.3.24.

Śrīśa Chandra Vasu, The Ashtadhyayi of Panini (1897), via ashtadhyayi.com.