विभाषर्जोश्छन्दसि

Operational rule vidhi

Before the affixes 1. इष्ठ 2. इमन् and 3. ईयस् , the ऋ of ऋजु may optionally be changed to र् , in the छन्दस् (Vedas).

Word by word (padaccheda)

Substitute / term prathamā · ekavacanaTarget ऋजोः ṣaṣṭhī · ekavacanaRight context छन्दसि saptamī · ekavacana

Substitute / term Prathamā (1st case): in operational rules, the substitute or the thing introduced; in definitions, the term or its referent.
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. Hover or tap one to see its card.

Full reading

ऋजोः 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)

ऋजोः → / छन्दसि optional

Replace ṛjoḥ with vibhāṣā when chandasi follows, optionally. Inherited words (anuvṛtti) may add further conditions.

Commentary

Before the affixes 1. इष्ठ 2. इमन् and 3. ईयस् , the ऋ of ऋजु may optionally be changed to र् , in the छन्दस् (Vedas).
As रजिष्ठः and ऋजिष्ठः, in रजिष्ठमेति पन्थानम्, त्वमृजिष्ठः ॥

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