OM, or AUM, is a cosmic sound, the sound of creation, the first sound in the universe.  The Sanskrit word aum means all; being present everywhere, all powerful, and all knowing.  The symbol for AUM is made up of the three syllables A, U, and M, which can be interpreted many ways.  The three letters can be thought of as the beginning, middle and end or the past, present, and future.  They symbolize speech, the mind, and the breath or the dimensions of length, breadth, and depth. They represent the three deities: Brahma (creator), Vishnu (preserver), and Shiva (destroyer).  They correspond to the three genders: masculine, feminine and neuter and to the waking state, dream state, and dreamless sleep state.  The crescent and dot in the written symbol signifies clearing the veil or, in Sanskrit, maya of illusion, allowing us to see who we truly are and clearing our mind and senses.  The entire symbol is said to represent the Creator, all of creation, the living spirit, or the perfect man.  Chanting OM in a yoga class brings the class together, unifying the energy in the room, clearing out blockages, and stimulating the heart chakra.  When the chanting of OM begins a Yoga practice, it should also seal the practice to maintain balance.