ध्वमो ध्वात्

Operational rule vidhi

In the छन्दस् (Vedas) ध्वात् is substituted for the personal ending ध्वम्.

Word by word (padaccheda)

Target ध्वमः ṣaṣṭhī · ekavacanaSubstitute / term ध्वात् prathamā · ekavacana

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

ध्वमः ध्वात्

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

As a rewrite rule (sketch from case endings)

ध्वमः → ध्वात्

Replace dhvamaḥ with dhvāt. Inherited words (anuvṛtti) may add further conditions.

Commentary

In the छन्दस् (Vedas) ध्वात् is substituted for the personal ending ध्वम्।
As अन्तरेवोष्माणं वारयध्वात् for वारयध्वम् ॥ Aitareya Brahmana II.6.14.

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