देवसुम्नयोर्यजुषि काठके

Operational rule vidhi

Long आ is substituted for the final of देव and सुम्न before the Denominative क्यच् in यजुष काठक.

Word by word (padaccheda)

Target देवसुम्नयोः ṣaṣṭhī · dvivacanaRight context यजुषि saptamī · ekavacanaRight context काठके saptamī · ekavacana

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

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

Full reading

देवसुम्नयोः यजुषि काठके

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

Commentary

Long आ is substituted for the final of देव and सुम्न before the Denominative क्यच् in यजुष काठक।
As देवायन्तो यजमानाः सुम्नायन्तो हवामहे ॥ Why in the Yajus? Observe देवाञ्जिगाय सुम्नयुः ॥ Why do we say in the Kathaka? Observe सुम्नयुरिदमासात् ॥

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