Conversations on Om
The letter 'A' denotes Balaram , the son of Rohini, who is the substratum of the entire universe, who is the supersoul of the universe. The letter 'M' denotes Anirudha, who is the supersoul of each individual being in the universe. And the 'dot' above the 'M' denotes Sri Krishna, the fountain head of all Vishnu incarnations.