दिशोऽमद्राणाम्

Operational rule vidhi

After a word denoting direction, the first vowel of the name of a country with the exception of मद्र gets वृद्धि before a तद्धित affix having an indicatory ञ् , ण् or क् .

Word by word (padaccheda)

Left context दिशः pañcamī · ekavacanaTarget अमद्राणाम् ṣaṣṭhī · bahuvacana

Left context Pañcamī (5th case): "after X", the operation applies to what follows (1.1.67 तस्मादित्युत्तरस्य). Also used for limits and sources.
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

दिशः अमद्राणाम्

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

Commentary

After a word denoting direction, the first vowel of the name of a country with the exception of मद्र gets वृद्धि before a तद्धित affix having an indicatory ञ् , ण् or क् ।
As पूर्वपाञ्चालकः, अपरपाञ्चालकः, दक्षिणपाञ्चालकः, उत्तरपाञ्चालकः (4.2.125, 4.2.107, 4.2.108. 6.2.105 accent). The Tadanta-vidhi applies here, as shown in the preceding sutra. Why do we say 'denoting direction'? Observe पूर्वः पाञ्चालानां = पूर्वपञ्चालः, तत्र भवः, = पौर्वपञ्चालकः, आपरपञ्चालकः ॥ With मद्र we have पौर्वमद्रः, आपरमद्रः with अञ् 4.2.108. The separation of this sutra from the last is for the sake of the subsequent sutra.

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