अर्मे चावर्णं द्व्यच्त्र्यच्

Operational rule vidhi

A word of two or three syllables ending in a अ or आ (with the exception of महा and नव) standing before the word अर्म has acute accent (उदात्त) on the first syllable.

Word by word (padaccheda)

Right context अर्मे saptamī · ekavacanaIndeclinable च avyayaSubstitute / term अवर्णम् prathamā · ekavacanaSubstitute / term द्व्यच् prathamā · ekavacanaSubstitute / term त्र्यच् prathamā · 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").
Indeclinable An avyaya such as च "and", वा "optionally", न "not".
Substitute / term Prathamā (1st case): in operational rules, the substitute or the thing introduced; in definitions, the term or its referent.

Dotted words are technical terms. Hover or tap one to see its card.

Full reading

अर्मे च अवर्णम् द्व्यच् त्र्यच्

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

Commentary

A word of two or three syllables ending in a अ or आ (with the exception of महा and नव) standing before the word अर्म has acute accent (उदात्त) on the first syllable.
Thus द꣡त्तार्मम्, गु꣡प्तार्मम्, कु꣡क्कुटार्मम्, वा꣡यसार्मम् ॥ Why do we say 'ending in अ (long or short)'? Observe बृहदर्मम् ॥ Why do we say 'consisting of two or three syllables'? Observe कपिञ्जलार्मम् ॥ The words महा and नव are to be read here also. The rule therefore does not apply to महार्मम् and नवार्मम् ॥

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