युप्लुवोर्दीर्घश्छन्दसि

Operational rule vidhi

In यु and प्लु , long is substituted for उ in the छन्दस् (Vedas).

Word by word (padaccheda)

Target यु- प्लुवोः ṣaṣṭhī · dvivacanaSubstitute / term prathamā · ekavacanaRight context छन्दसि saptamī · ekavacana

Target Ṣaṣṭhī (6th case): names what gets replaced, "in place of X" (1.1.49 षष्ठी स्थानेयोगा).
Substitute / term Prathamā (1st case): in operational rules, the substitute or the thing introduced; in definitions, the term or its referent.
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. Hover or tap one to see its card.

Full reading

यु-प्लुवोः छन्दसि

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

As a rewrite rule (sketch from case endings)

यु- प्लुवोः → / छन्दसि

Replace yu-pluvoḥ with dīrghaḥ when chandasi follows. Inherited words (anuvṛtti) may add further conditions.

Commentary

In यु and प्लु , long is substituted for उ in the छन्दस् (Vedas).
Thus दान्त्यनुपूर्वं वियूय; यत्रा यो दक्षिणा परिप्लूय ॥ Why do we say 'in the Chhandas'? Observe संपुत्य, आप्लुत्य in the secular literature.

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