रायो हलि

Operational rule vidhi

The word 'रै' gets an आकारादेश when followed by हलादि विभक्तिप्रत्यय.

Word by word (padaccheda)

Target रायः 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

रायः

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

Commentary

Before a case-ending beginning with a consonant, आ is substituted for the final of रै।
As राभ्याम्, राभिः ॥ But रायौ, रायः before affixes beginning with a vowel. Why 'a case ending'? Observe रैत्वम्, रैता ॥

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