Made from Aa, Oo and Am. They represent Vishnu, Shiva and Brahma respectively as per Vayupurana. Om means Brahmam. All mantras begin with Om. Because of its special powers, it was not supposed to be spoken or heard by the lower castes. The word contains everything that has happened and will happen as per Mandukopanishad. When the creator realised that the world was immersed in impure water, he split it open and from it emerged Omkara. The first sound that emerged was Bhu, the second Bhuva and last Swaha, and together they became BhurBhuvaSwaha. As per the Chhandogyopanishad, the earth is the summary of all living beings, and water is the summary of earth, water-plants – man, and man’s summary is his speech. The summary of speech is the Rigveda, its summary is the Samaveda and its summary is the Udgidha or Samavedadhwani and that is Om.