O: The Siddham Letter

O is the first strengthening grade of u, standing to that vowel exactly as e stands to i, and like e it is always long in the classical language. The symmetry is not decorative. It means that a reader who meets this vowel at the front of a word is usually looking at a form built from a root containing u, and the grammar of the derivation is recoverable from the letter alone.

Historically the sound is what a and u produce when they combine, which is also why it turns up constantly at word joins. A word ending in short a followed by one beginning in u is written with this single letter, so a good proportion of the o sounds on any page are seams rather than vowels belonging to either word. Undoing the seam is part of reading, and it is the reason a Siddham text is harder to parse than its letter count suggests.

The letter carries weight far beyond its ordinary frequency because of the syllable Om, which opens and closes an enormous quantity of what was ever written in this script. The seed syllables that East Asian esoteric Buddhism took from the table are built from vowels of this kind rather than from running words, and the vowel section of the syllabary is therefore the part of it a non-reader is most likely to have seen.