वृद्धात् प्राचाम्

Operational rule vidhi

The affixes ठञ् comes in the remaining senses after a वृद्ध word ending in उ and denoting a locality of East-folks.

Word by word (padaccheda)

Left context pañcamī · ekavacanaTarget प्राचाम् 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).

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Full reading

प्राचाम्

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

Commentary

The affixes ठञ् comes in the remaining senses after a वृद्ध word ending in उ and denoting a locality of East-folks.
The phrase ओर्देशी is here understood. Thus आढकजम्बुकः, शाकजम्बुकः नापितवास्तुकः ॥

The affix ठञ् would have been valid by the last sutra also, the present sutra makes a नियम or restriction, showing that in the case of words denoting places of East, the rule applies to those words only, which are Vriddha. Therefore it does not apply to मल्लवास्तु, from which we have माल्लवास्तवः ॥

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