वा यौ

Operational rule vidhi

वी is optionally the substitute of अज् when the affix -- अनद्यतने लुट् (3.3.15) etc. follows.

Word by word (padaccheda)

Indeclinable वा avyayaRight context यौ saptamī · ekavacana

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

वा यौ

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

Commentary

वी is optionally the substitute of अज् when the affix -- अनद्यतने लुट् 3.3.15 etc. follows.
This declares an option in case of the affix ल्युट्, the word यु in the Sutra referring to this affix; as, प्रवयणः or प्राजनो दण्डः 'driving stick or whip.'

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