तुश्छन्दसि

Operational rule vidhi

The affixes इष्ठन् and इयसुन् come in the छन्दस् (Vedas) after a प्रातिपदिक (Nominal Stem) ending in तृ.

Word by word (padaccheda)

Left context तुः pañcamī · ekavacanaRight context छन्दसि saptamī · ekavacana

Left context Pañcamī (5th case): "after X", the operation applies to what follows (1.1.67 तस्मादित्युत्तरस्य). Also used for limits and sources.
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

तुः छन्दसि

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

Commentary

The affixes इष्ठन् and इयसुन् come in the छन्दस् (Vedas) after a प्रातिपदिक (Nominal Stem) ending in तृ।
The तुः is ablative of तृ which includes both तृन् and तृच् ॥ This extends the application of these affixes to words other than attributes. Thus अतिशयेन कर्ता = करिष्ठः (कर्तृ + इष्ठन् = कर् + इष्ठ by 6.4.154, तृ is elided) As आसुर्ति करिष्ठः, दोहीयसी धेनुः ॥ By Vartika under 6.3.35 दोहित्री is changed into masculine दोहितृ and then ईयस् is added, when तृ is elided by 6.4.154.

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