ऊदनोर्देशे

Operational rule vidhi

After अनु , ऊ is substituted for (the अ of) अप् when the sense is that of a locality.

Word by word (padaccheda)

Substitute / term ऊत् prathamā · ekavacanaLeft context अनोः pañcamī · ekavacanaRight context देशे saptamī · ekavacana

Substitute / term Prathamā (1st case): in operational rules, the substitute or the thing introduced; in definitions, the term or its referent.
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").

Full reading

ऊत् अनोः देशे

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

Commentary

After अनु , ऊ is substituted for (the अ of) अप् when the sense is that of a locality.
Thus अनूपो देशः, but अन्वीपम् when locality is not meant. The long ऊ is taught for the sake of showing how the word is to be analysed; as अनु ऊपः = अनूपः ॥ The form अनूपः could have been evolved with a short उ also, as अनु + उपः = अनूपः ॥ But this analysis is not intended.

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