ब्रह्मणो जानपदाख्यायाम्

Operational rule vidhi

The affix टच् comes after the word ब्रह्मण in a तत्-पुरुष समास when the compound denotes a person belonging to a certain country.

Word by word (padaccheda)

Left context ब्रह्मणः pañcamī · ekavacanaRight context जानपदाख्यायाम् saptamī · ekavacana

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").

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Full reading

ब्रह्मणः जानपदाख्यायाम्

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

Commentary

The affix टच् comes after the word ब्रह्मण in a तत्-पुरुष समास when the compound denotes a person belonging to a certain country.
The word जानपदः = जनपदेषु भवः 'belonging to a country'. This affix comes after that Tatpurusha the first member of which is a word denoting a country. Thus सुराष्ट्रेषु ब्रह्मा = सुराष्ट्रब्रह्मः, and अवन्तिब्रह्मः "a Brahmana of Surashtra or Avanti". This is an example of Locative Tatpurusha compound.

Why do we say "when referring to the name of a country"? Observe देवब्रह्मा नारदः 'Narada the bard of the gods'.

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