O: The Vowel Inside Om

O is the thirteenth letter of the varnamala, the combination vowel that results when a and u come together, and the counterpart on the labial side to what e is on the palatal. Like the other three sandhyakshara it is always long and always heavy in metre, and it has no short partner in the series. Its vowel sign is compound, an upright stroke set to the right of the consonant together with a stroke above the headline, so it borrows from the way long a is written and from the way e is written and combines the two. The letter has more visibility in religious usage than any other combination vowel because Om is written with it. The syllable is a and u combined into o, with the nasal closing it, which is precisely why the tradition that takes Om apart into three sounds is able to do so: the analysis into a, u and m is a reading of the syllable back through the ordinary rule of sandhi that produced it.

The Mandukya Upanishad is the text that develops that analysis furthest, correlating a with waking, u with dream and m with deep sleep, and the silence beyond them with a fourth state it names only as the fourth. That scheme belongs to the Upanishad and to its long tradition of commentary, and it is recorded here as theirs.