मित्रे चर्षौ

Operational rule vidhi

The final vowel of विश्व is lengthened before मित्र when it is the name of a ऋषि.

Word by word (padaccheda)

Right context मित्रे saptamī · ekavacanaIndeclinable च avyayaRight context ऋषौ saptamī · 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").
Indeclinable An avyaya such as च "and", वा "optionally", न "not".

Dotted words are technical terms. Hover or tap one to see its card.

Full reading

मित्रे च ऋषौ

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

Commentary

The final vowel of विश्व is lengthened before मित्र when it is the name of a ऋषि।
As विश्वामित्रः 'the sage Visvamitra'. But विश्वमित्रः 'a boy called Visvamitra'.

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