This article records tradition as it has been passed down and reported. Its sources are not yet part of the atlas's verified catalogue.
Siddham Letters gathers the forty seven signs of the abugida that carried Sanskrit Buddhist scripture into China and Japan, a script whose table of vowels and consonants is inherited whole from the Brahmi tradition and whose devotional use in East Asia outlasted its use in India.
Every consonant sign carries the inherent vowel A, so the independent letter A stands not as one sound among the others but as the ground they are all read against, the least shaped sound the Sanskrit phoneticians could describe. The table pairs every simple vowel with a long counterpart, adds the vocalic liquids R and L in short and long forms though the long L occurs in no attested Sanskrit word at all, and closes its compound vowels and its five classes of consonant exactly as the Devanagari and Brahmi tables do, since all three set down the same analysis of Sanskrit in different letterforms.
In the esoteric Buddhist traditions that carried Siddham east, the syllable A is read as the mark of the unborn, and contemplating the written letter is a named practice in Japanese Shingon, one of the clearest cases in the atlas of a script's letters carrying devotional weight beyond their phonetic value. Centuries of hand copying across India, China and Japan left variant shapes for several letters, recorded in the character standard as alternate two circle and three circle forms, and the study of the script became a scholarly discipline of its own in Japan, one that preserved even the letters a working scribe would rarely have occasion to write.