पादस्य लोपोऽहस्त्यादिभ्यः

Operational rule vidhi

The final अ of पाद is elided when it is at the end of a बहुव्रीहि समास preceded by a word denoting a thing with which it is compared but not so when such word is हस्ति etc.

Word by word (padaccheda)

Target पादस्य ṣaṣṭhī · ekavacanaTarget ṣaṣṭhī · ekavacanaLeft context अहस्त्यादिभ्यः pañcamī · bahuvacana

Target Ṣaṣṭhī (6th case): names what gets replaced, "in place of X" (1.1.49 षष्ठी स्थानेयोगा).
Left context Pañcamī (5th case): "after X", the operation applies to what follows (1.1.67 तस्मादित्युत्तरस्य). Also used for limits and sources.

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

Full reading

पादस्य अहस्त्यादिभ्यः

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

Commentary

The final अ of पाद is elided when it is at the end of a बहुव्रीहि समास preceded by a word denoting a thing with which it is compared but not so when such word is हस्ति etc.
The word उपमानात् is understood here. The लोप substitution is a samasanta affix in a way. Thus व्याघ्रस्येव पादावस्य = व्याघ्रपात्, सिंहपात् ॥ But हस्तिपादः, कटोलपादः ॥

1 हस्तिन्, 2 कुदाल, 3 अश्व, 4 कशिक, 5 कुरुत, 6 कटोल, 7 कटोलक, 8 गण्डोल, 9 गण्डोलक, 10 कण्डोल, 11 कण्डोलक, 12 अज, 13 कपोत , 14 जाल, 15 गण्ड, 16 महेल, (महिला), 17 दासी, 18 गणिका, 19 कुसूल ॥

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