Mandukya Upanishad Verse 8
Sanskrit Verse
सोऽयमात्माध्यक्षरमोङ्कारोऽधिमात्रं पादा मात्रा मात्राश्च पादा अकार उकारो मकार इति ॥ ८ ॥
Transliteration
so’yamātmādhyakṣaramoṅkāro’dhimātraṃ pādā mātrā mātrāśca pādā akāra ukāro makāra iti || 8 ||
Translation
The same Ātman (which has been described above as having four quarters) is, again, Aum, from the point of view of the syllables (akṣaram). The Aum with parts is viewed from the standpoint of sounds (letters, mātrāḥ). The quarters are the letters (parts) and the letters are the quarters. The letters here are A, U and M.

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