Why is Omkar called as pranav mantra

Why is Omkar called as pranav mantra ("monarch of mantras") ?

This is written as ‘Om (ॐ)’ and is considered as the sacred vowel. Not only is Omkār a Mahāmantra with a single letter but is also labeled as the monosyllabic Brahman in the Shrīmadbhagwadgitā. The Sage of this mantra is Parabrahman, the Parmātmā and the rhythm (chanda) Gāyatrī. The Mahamantras from the Vēdās, Upanishads, etc. and also the efficacious (Siddha) mantras and Names denoting The God are the varied forms of this Omkar. When chanting every mantra it is essential to start it with Omkar.

‘The scriptures describe the pranav, that is, Om as the “monarch of mantras”. It is considered as the symbol or representation of the individual soul. Omkar is the main Name of The Supreme Brahman (Parabrahman). Since acquisition of knowledge about it means realisation of Parabrahman, worshipping it is the same as worshipping The Supreme God. The famous quote from the Upanishads ‘अयमात्‍मा ब्रह्म’ means ‘This soul is Brahman’. Once one realises through meditation that the entire universe is composed of Omkar and develops conviction that every object in the universe and every individual soul itself is Brahman, the realisation of Brahman as the soul, is achieved and finally one attains Self-realisation of the unmanifest (Nirgun) Brahman in the form of “I am Brahman (अहं ब्रह्मास्‍मि)”.

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