A deeper insight into this mystic symbol reveals that it is composed of three syllables combined into one, not like a physical mixture but more like a chemical combination. Indeed in Sanskrit the vowel 'o' is constitutionally a diphthong compound of a + u; hence OM is representatively written as AUM.
Fittingly, the symbol of AUM consists of three curves (curves 1, 2, and 3), one semicircle (curve 4), and a dot.
The large lower curve 1 symbolizes the waking state (jagrat), in this state the consciousness is turned outwards through the gates of the senses. The larger size signifies that this is the most common ('majority') state of the human consciousness.
The upper curve 2 denotes the state of deep sleep (sushupti) or the unconscious state. This is a state where the sleeper desires nothing nor beholds any dream.
The middle curve 3 (which lies between deep sleep and the waking state) signifies the dream state (swapna). In this state the consciousness of the individual is turned inwards, and the dreaming self beholds an enthralling view of the world behind the lids of the eyes.
These are the three states of an individual's consciousness, and since Indian mystic thought believes the entire manifested reality to spring from this consciousness, these three curves therefore represent the entire physical phenomenon.
The dot signifies the fourth state of consciousness, known in Sanskrit as turiya. In this state the consciousness looks neither outwards nor inwards, nor the two together. It signifies the coming to rest of all differentiated, relative existence This utterly quiet, peaceful and blissful state is the ultimate aim of all spiritual activity. This Absolute (non-relative) state illuminates the other three states.
Finally, the semi circle symbolizes maya and separates the dot from the other three curves. Thus it is the illusion of maya that prevents us from the realization of this highest state of bliss. The semi circle is open at the top, and does not touch the dot. This means that this highest state is not affected by maya. Maya only affects the manifested phenomenon. This effect is that of preventing the seeker from reaching his ultimate goal, the realization of the One, all-pervading, unmanifest, Absolute principle. In this manner, the form of OM represents both the unmanifest and the manifest, the noumenon and the phenomenon.
As a sacred sound also, the pronunciation of the three-syllabled AUM is open to a rich logical analysis.
The first alphabet A is regarded as the primal sound, independent of cultural contexts. It is produced at the back of the open mouth, and is therefore said to include, and to be included in, every other sound produced by the human vocal organs. Indeed A is the first letter of the Sanskrit alphabet.
The open mouth of A moves toward the closure of M. Between is U, formed of the openness of A but shaped by the closing lips. Here it must be recalled that as interpreted in relation to the three curves, the three syllables making up AUM are susceptible to the same metaphorical decipherment. The dream state (symbolized by U), lies between the waking state (A) and the state of deep sleep (M). Indeed a dream is but the compound of the consciousness of waking life shaped by the unconsciousness of sleep.
AUM thus also encompasses within itself the complete alphabet, since its utterance proceeds from the back of the mouth (A), travelling in between (U), and finally reaching the lips (M). Now all alphabets can be classified under various heads depending upon the area of the mouth from which they are uttered. The two ends between which the complete alphabet oscillates are the back of the mouth to the lips; both embraced in the simple act of uttering of AUM.
The last part of the sound AUM (the M) known as ma or makar, when pronounced makes the lips close. This is like locking the door to the outside world and instead reaching deep inside our own selves, in search for the Ultimate truth.
But over and above the threefold nature of OM as a sacred sound is the invisible fourth dimension which cannot be distinguished by our sense organs restricted as they are to material observations. This fourth state is the unutterable, soundless silence that follows the uttering of OM. A quieting down of all the differentiated manifestations, i.e. a peaceful-blissful and non-dual state. Indeed this is the state symbolized by the dot in the traditional iconography of AUM.
The threefold symbolism of OM is comprehensible to the most 'ordinary' of us humans, realizable both on the intuitive and objective level. This is responsible for its widespread popularity and acceptance. That this symbolism extends over the entire spectrum of the manifested universe makes it a veritable fount of spirituality.
Om - An Inquiry into its Aesthetics, Mysticism, and Philosophy