The Sages gave to the Supreme the name A–U–M which stands as the root of all language. The first letter “A” is the mother–sound, being the natural sound uttered by every creature when the throat is opened, and no sound can be made without opening the throat. The last letter “M,” spoken by closing the lips, terminates all articulation. As one carries the sound from the throat to the lips, it passes through the sound “U.” These three sounds therefore cover the whole field of possible articulate sound. Their combination is the most universal name which can be given to the Supreme.
~ Katha Upanishad