तिरसस्तिर्यलोपे

Operational rule vidhi

If the word 'तिरस्' is followed by a लुप्तप्रत्ययान्त-अञ्च् word from which an अकार has not been removed, then the word 'तिरस्' is converted to 'तिरि'.

Word by word (padaccheda)

Target तिरसः ṣaṣṭhī · ekavacanaWord तिरि Right context अलोपे saptamī · ekavacana

Target Ṣaṣṭhī (6th case): names what gets replaced, "in place of X" (1.1.49 षष्ठी स्थानेयोगा).
Word Case not marked in the source data.
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

तिरसः तिरि अलोपे

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

Commentary

तिरि is substituted for तिरस् before this अञ्च् when no affix (or व affix) follows, provided that the अ of अञ्च् is not elided.
Thus तिर्यङ्, तिर्यञ्चौ, तिर्यञ्चः ॥ Why do we say 'when there is no elision'? Observe तिरश्चा, तिरश्चे ॥ Here अ is elided by 6.4.138. The word अलोपे in the sutra is ambiguous. It may mean either (1) "where अ is elided" or (2) "Where there is no elision". The latter meaning should be taken however.

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