व्युपयोः शेतेः पर्याये

Operational rule vidhi

The affix घञ् comes after the root सी 'to lie down' when the prepositions वि and उप are in composition with it and when the sense of the word so formed is that of 'regular succession'.

Word by word (padaccheda)

Right context वि- उपयोः saptamī · dvivacanaLeft context शेतेः pañcamī · ekavacanaRight context पर्याये saptamī · 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").
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

वि-उपयोः शेतेः पर्याये

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

Commentary

The affix घञ् comes after the root सी 'to lie down' when the prepositions वि and उप are in composition with it and when the sense of the word so formed is that of 'regular succession'.
As, तव विशायः 'thy turn of sleeping' (i.e. the rest enjoyed in rotation by the sentinels on watch); मम विशायः 'my turn of rest'; तव राजोपशायः राजानमुपशयितुम्पर्यायः 'thy turn of sleeping the king'.

Why do we say 'regular succession'? Observe उपशयः 'lying by the side of or ambush'; विशयः 'doubt'.

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