पुत्रः पुंभ्यः

Operational rule vidhi

The word पुत्र coming after a Masculine noun in a तत्-पुरुष समास has a acute accent (उदात्त) on the first syllable.

Word by word (padaccheda)

Substitute / term पुत्रः prathamā · ekavacanaLeft context पुम्भ्यः pañcamī · bahuvacana

Substitute / term Prathamā (1st case): in operational rules, the substitute or the thing introduced; in definitions, the term or its referent.
Left context Pañcamī (5th case): "after X", the operation applies to what follows (1.1.67 तस्मादित्युत्तरस्य). Also used for limits and sources.

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Full reading

पुत्रः पुम्भ्यः

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

Commentary

The word पुत्र coming after a Masculine noun in a तत्-पुरुष समास has a acute accent (उदात्त) on the first syllable.
Thus कौनटिपु꣡त्रः, दामकपु꣡त्रः, माहिषपु꣡त्रः ॥ Why do we say 'a पुत्र'? Observe कौनटिमातुलः ॥ Why do we say 'after a masculine word'? Observe गार्गीपुत्रः, वात्सीपुत्रः ॥

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