बह्वचः कूपेषु

Operational rule vidhi

In the context of चतुरर्थाः, to indicate the name of a well by referring to a word that has three or more स्वराः , अञ् प्रत्यय is used.

Word by word (padaccheda)

Left context बहु- pañcamī · ekavacanaRight context कूपेषु saptamī · bahuvacana

Left context Pañcamī (5th case): "after X", the operation applies to what follows (1.1.67 तस्मादित्युत्तरस्य). Also used for limits and sources.
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, dashed ones pratyāhāras (names for classes of sounds or affixes). Hover or tap one to see its card.

Full reading

बहु- कूपेषु

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

Commentary

The affix अञ् comes in the quadruple sense after a polysyllabic प्रातिपदिक (Nominal Stem) when 'a well' is to be designated.
This debars अण्. Thus दीर्घवरत्रेण निर्वृत्तः कूपः = दैर्घवरत्रः "a well completed by Dirghavaratra". So also कापिलवरत्रः ॥

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