Tantra Sāra 1.013
nārāyaṇāṣṭākṣarācca vyāhr̥tibhyastathaiva ca। vibhēdō dvādaśārṇānāṁ kēśavādyāśca dēvatāḥ ॥13॥
From the eight letter Narayana mantra and the four Vyahrtis, in order, manifests the twelve letter mantra. From it comes the twelve forms of the lord such as Kēshava and others.
Note:
The tweleve letter mantra, "Ōm Na-Mō Bha-Ga-Va-Tē Vā-Su-Dē-Vā-Ya" also comes from eight syllables of Pranava i.e, Ā, Ū, M, Nāda = sound, Bindu = pause, Ghosha = roar, Shanta = calm, Ati-Shanta = intense calmness, and four vyahritis, i.e. 'Bhūhu', 'Bhuvaha', 'Suvaha', and 'Bhūrbhuvahsvaha'.
A pictorial representation:
