तनिपत्योश्छन्दसि

Operational rule vidhi

In the छन्दस् (Vedas) the root-vowel of तन् and पत् is elided before an affix beginning with a vowel, when it has an indicatory क् or ङ् .

Word by word (padaccheda)

Target तनि- पत्योः ṣaṣṭhī · dvivacanaRight context छन्दसि 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, dashed ones pratyāhāras (names for classes of sounds or affixes). Hover or tap one to see its card.

Full reading

तनि-पत्योः छन्दसि

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

Commentary

In the छन्दस् (Vedas) the root-vowel of तन् and पत् is elided before an affix beginning with a vowel, when it has an indicatory क् or ङ् ।
As वितत्निरे कवयः, शकुना इव पप्तिमः ॥ In secular language वितेनिरे, पेतिम् ॥

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