In the esoteric Buddhism of Japan, Siddham is not a historical curiosity but working ritual equipment. Its principal use is writing bija, seed syllables, single characters that condense a buddha, a bodhisattva or a truth into one written sound: A for the primordially unborn, associated with Dainichi in the Womb realm, HUM for Fudo Myoo, HRIH for Amida, KIRIK in Japanese usage for Kannon's syllable.
In Shingon meditation the practitioner visualises the syllable, often standing on a lotus within a moon disc, and the letter itself is contemplated as the deity's body. The A-syllable meditation, ajikan, takes the single Siddham letter A as the object of the entire practice. Siddham letters appear on gravestones and stupas, on ofuda and omamori, and at the head of mandalas, where each deity may be represented by its syllable rather than its image. Mantras and dharani are copied in Siddham as devotional calligraphy, with brush styles and stroke orders transmitted from master to student. Because the tradition regards the exact form of the written syllable as effective, scribal accuracy is itself a religious duty, and manuals of shittan calligraphy have been produced in Japan continuously since the Heian period.