पुरे प्राचाम्

Operational rule vidhi

Before the word पुर , when the compound denotes a city of the Eastern People, the final of the final of the first member as the acute accent (उदात्त).

Word by word (padaccheda)

Right context पुरे saptamī · ekavacanaTarget प्राचाम् ṣaṣṭhī · bahuvacana

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").
Target Ṣaṣṭhī (6th case): names what gets replaced, "in place of X" (1.1.49 षष्ठी स्थानेयोगा).

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Full reading

पुरे प्राचाम्

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

Commentary

Before the word पुर , when the compound denotes a city of the Eastern People, the final of the final of the first member as the acute accent (उदात्त).
Thus ललाट꣡पुरम्, कांची꣡पुरम्, शिवदत्त꣡पुरम्, काणि꣡पुरम्, नामपुरम् ॥ Why do we say 'of the Eastern people'? Observe शिवपुरम् ॥

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