रधिजभोरचि

Operational rule vidhi

The verb roots रध् and जभ् get a नुमागम when followed by an अजादि प्रत्यय.

Word by word (padaccheda)

Target रधि- जभोः ṣaṣṭhī · dvivacanaRight 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

रधि-जभोः

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

Commentary

The augment नुम् is added after the root-vowel in रध् and जभ् before affixes beginning with a consonant.
Thus रन्धयति, रन्धकः, साधुरन्धी, रन्धो वर्तते ॥ जम्भयति, जम्भकः, साधुजम्भी, जम्भंजम्भम्, जम्भो वर्तते ॥ Though the वृद्धि rule is subsequent, it is superseded by the augment. Why do we say 'before an affix beginning with a vowel'? Observe रद्धा, जभ्यम् ॥

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