वत्सोक्षाश्वर्षभेभ्यश्च तनुत्वे

Operational rule vidhi

The affix ष्टरच् expresses slenderness after the words 1. वत्स 2. उक्षन् 3. अश्व and 4. ऋषव.

Word by word (padaccheda)

Left context वत्स- उक्ष- अश्व- ऋषभेभ्यः pañcamī · bahuvacanaIndeclinable च avyayaRight context तनुत्वे 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.
Indeclinable An avyaya such as च "and", वा "optionally", न "not".
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").

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Full reading

वत्स-उक्ष-अश्व-ऋषभेभ्यः च तनुत्वे

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

Commentary

The affix ष्टरच् expresses slenderness after the words 1. वत्स 2. उक्षन् 3. अश्व and 4. ऋषव।
The anuvritti of हृस्व ceases. Thus वत्सतरः 'a weaned calf' becoming slender in reaching the second period of its age. उक्षतरः 'a slender bull': being the third age of a वत्स ॥ अश्वतरः 'a mule': (तस्य तनुत्वमन्यपितृकता); वृषभतरः 'a slender bull or that cannot draw the load'.

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