7.2.100
अचि र ऋतः
Operational rule vidhi
The ऋकार of the words तिसृ and चतसृ is converted to रेफ when followed by an अजादि-विभक्तिप्रत्यय.
Word by word (padaccheda)
Right context saptamī · ekavacanaSubstitute / term रः prathamā · ekavacanaTarget ऋतः ṣaṣṭhī · ekavacana
- 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").
- 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).
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
रः ऋतः
Pāṇini leaves out words that carry over from earlier sūtras. Expand to see the rule as a reader actually understands it.
ऋतः → रः /
Replace ṛtaḥ with raḥ when aci follows. Inherited words (anuvṛtti) may add further conditions.
Commentary
र् is substituted for ऋ of तिसृ and चतसृ before the case-affixes beginning with a vowel.
Thus तिस्रः, (in तिस्रस्तिष्ठन्ति, तिस्रः पश्य) चतस्रस्तिष्ठन्ति, चतस्रः पश्य ॥ प्रियतिस्र आनय, प्रियचतस्र आनय, प्रियतिस्रः त्वम्, प्रियचतस्रः त्वम् ॥ प्रियतिस्रि निधेहि, प्रियचतस्रि निधेहि ॥ This supersedes 6.1.102 (ordaining the single long substitution of the form of the first), 6.1.111 (substitution of उर् for ॠ), and 7.3.110 (the substitution of guna). The last rule 7.3.110, though subsequent in order, is however superseded by this rule. Why 'before the affixes beginning with a vowel?' Observe तिसृभिः, चतसृभिः ॥ The ॠतः of the sutra refers to the ॠ of तिसृ and चतसृ and not to a ॠ ending stem in general: and had it not been used in the sutra, the latter would have stood thus अचिरः "र् is substituted for the final of the words above-mentioned before an affix beginning with a vowel." So र् would have been substituted for the final of त्रि and चतुर् also.
Śrīśa Chandra Vasu, The Ashtadhyayi of Panini (1897), via ashtadhyayi.com.