विश्वस्य वसुराटोः

Operational rule vidhi

The final vowel of विश्व is lengthened before वसु and राट् (the form assumed by राज्).

Word by word (padaccheda)

Target विश्वस्य ṣaṣṭhī · ekavacanaRight context वसु- राटोः saptamī · dvivacana

Target Ṣaṣṭhī (6th case): names what gets replaced, "in place of X" (1.1.49 षष्ठी स्थानेयोगा).
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

विश्वस्य वसु-राटोः

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 वसु and राट् (the form assumed by राज्).
Thus विश्वावसुः, विश्वाराट् ॥ The rule applies to the राट् form of राज in the Nominative singular, and not when it retains its own form: as विश्वराजौ, विश्वराजः ॥

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