भुवश्च महाव्याहृतेः

Operational rule vidhi

In the छन्दस् (Vedas), both रु and र् may replace the final of the word भुवस् when used as a महाव्याहृति.

Word by word (padaccheda)

Word भुवः Indeclinable च avyayaTarget महाव्याहृतेः ṣaṣṭhī · ekavacana

Word Case not marked in the source data.
Indeclinable An avyaya such as च "and", वा "optionally", न "not".
Target Ṣaṣṭhī (6th case): names what gets replaced, "in place of X" (1.1.49 षष्ठी स्थानेयोगा).

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Full reading

भुवः च महाव्याहृतेः

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

Commentary

In the छन्दस् (Vedas), both रु and र् may replace the final of the word भुवस् when used as a महाव्याहृति।
Thus भुवरित्यन्तरिक्षम् or भुव इत्यन्तरिक्षम् ॥ The maha-vyahritis are three, used generally before the famous Gayatri mantra. They denote respectively the earth, the firmament and the heaven. भुवस् is an Indeclinable and a Vyahriti denoting the firmament. The other two are भूः and स्वः ॥

Why do we say when it is a maha-vyahriti? Observe भुवो विश्वेषु भुवनेषु यज्ञियः ॥ Here भुवः is a verb, 2nd Person Singular, Imperfect (लङ्) of the root भू, without guna of the root, and the अट् augment is not added as a Vedic diversity.

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