सः स्यार्द्धधातुके

Operational rule vidhi

A सकारान्त अङ्ग gets a तकारादेश When followed by a सकारादि-आर्धधातुक प्रत्यय.

Word by word (padaccheda)

Target सः ṣaṣṭhī · ekavacanaRight context सि saptamī · ekavacanaRight context आर्द्धधातुके saptamī · ekavacana

Target Ṣaṣṭhī (6th case): names what gets replaced, "in place of X" (1.1.49 षष्ठी स्थानेयोगा).
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").

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

Full reading

सः सि आर्द्धधातुके

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

Commentary

त् is substituted for the final स् of a root before an आर्द्धधातुक affix beginning with a स्।
As वत्स्यति, अवत्स्यत, विवत्सति, जिघत्सति ॥ Why do we say 'when ending in स'? Observe वक्ष्यति ॥ Why do we say 'beginning with स'? Observe घासः, वासः ॥ Why an ardhadhatuka? Observe आस्से, वस्से, from आस् 'to sit' and वस् 'to cover'; both Adadi roots.

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