रोः सुपि

Operational rule vidhi

In presence of the सुप् प्रत्यय of सप्तमी-बहुवचन, a पदान्त र्-वर्ण is converted to a विसर्ग only if that र् is generated from रुँ, and not otherwise.

Word by word (padaccheda)

Target रोः ṣaṣṭhī · ekavacanaRight context saptamī · ekavacana

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").

Dotted words are technical terms, dashed ones pratyāhāras (names for classes of sounds or affixes). Hover or tap one to see its card.

Full reading

रोः

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

Commentary

The विसर्जनीय is substituted for रु called र (and not any other र) before the Locative -7th case plural case-affix सु।
Thus पयःसु, सर्प्पिःषु, यशःसु ॥ The word सुप् is here the Locative Plural affix. Though the र् would have been changed to visarga by the last sutra also; the making of this a special sutra is for the sake of niyama. That is, only रु becomes visarga, and not any other र् ॥ Thus गीर्षु, भूर्षु, when the र् is not रु ॥ In पयस् &c, the स् becomes रु by 8.2.66.

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