It is the Alpha or Omega.
Just as the mouth opens for the A,
so the process of creation is opened by this word.
It is the maintained by the U and comes to a close with the M.
Similarly AUM symbolizes the three relative states of waking, dreaming and deep sleep.
Yet beyond these,OM also has fourth a soundless aspect.
In this aspect, it symbolizes the a absolute silence from which the word is born.