तद्धितेष्वचामादेः

Operational rule vidhi

When followed by a णित् / ञित् तद्धित-प्रत्यय, the आदि-स्वर of the अङ्ग undergoes a वृद्धि.

Word by word (padaccheda)

Right context saptamī · bahuvacanaTarget ṣaṣṭhī · bahuvacanaTarget आदेः ṣaṣṭhī · ekavacana

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

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

आदेः

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

Commentary

The वृद्धि is substituted for the first vowel of the अङ्ग (stem) when a तद्धित affix having an indicatory ञ् or ण् follows.
As गार्ग्यः from गर्ग + यञ्, so also वात्स्यः ॥ दाक्षिः (दक्ष + इञ्), प्लाक्षिः, औपगवः (with अण् from उपगु), कापठवः &c. This debars the Vriddhi of 7.2.115 and 7.2.116 as त्वाष्ट्रः from त्वष्ट्, and जागतः from जगत् ॥

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