This single syllable...

"The sound A is the mother of all letters; it is the essence of all sounds; and it stands for the fountainhead of all-inclusive Reality... ...If we see the letter A, we know all things are empty and nil..." (Hakeda, p. 247)

This syllable is a symbolic representation of Mahavairocana as he appears in the Taizo-kai Mandala. It is also a central symbol in one of Shingon's most common but important meditations. Like the image of Mahavairocana, the syllable A is a universal symbol that excludes nothing. It represents all the teachings of the Dharma and, as stated above, the empty nature of all things and the ultimate reality that exists simultaneously.

The image here, often referred to as the "Aji" image depicts the syllable A resting on the lotus of great compassion and held within the full moon sphere of Bodhicitta. Please enjoy this elegant image.

"Just as we hear the sound A when we hear all sounds, so we perceive that which is the limit, the "original uncreated," when we perceive the arising of phenomena. He who perceives that which is the limit, the "originally uncreated," will come to know his mind as it really is. To know one's mind as it really is is [to gain] all-inclusive wisdom. Thus, this single letter is the mantra of Mahavairocana." (Hakeda, p. 249)

(Source: "The Meaning of the Word HUM", Ungi gi, as translated in "Kukai: Major Works", by Yoshito S. Hakeda, Columbia Press 1972)