In the chandas (Vedas), both ru and r may replace the final of the word bhuvas when used as a mahāvyāhṛti,
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.,