तोः षि

Operational rule vidhi

A तवर्ग letter which is followed by a षकार does not undergo ष्टुत्वम्.

Word by word (padaccheda)

Target तोः ṣaṣṭhī · ekavacanaRight context षि 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").

Full reading

तोः षि

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

Commentary

In the room of तु there is not a cerebral substitute, when ष् follows.
The word न is to be read into the aphorism. As, अग्निचित् + षण्डे = अग्निचित्षण्डे ॥ भवान्षण्डे, महान्षण्डे ॥

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