“A is the first and most important of the Siddham characters. . . .
    Meditation on the letter A (ajikan) is an essential practice of esoteric Buddhism. Usually, the character is set on an eight-petaled lotus in the center of a round moon, and then mounted on a board or scroll. The mounted character is set against a wall in the meditation hall. After performing the prescribed ritual including prostrations, mantras, and mudras, the meditator sits in the lotus posture about one meter from the character.
    Pronouncing the A sound with each breath, he visualizes the moon, drawing it into his heart and expanding it gradually until its brilliance permeates the universe. Now from the center of the moon A is perceived as the essence of Mahavairocana—the distinction between worshiper and worshiped is effaced. Near the end of the meditation the moon and the letter are returned to their original form.”

—John Stevens, Sacred Calligraphy of the East, third edition, 1995.