The Mandukya Upanishad is set out in twelve verses and takes the syllable Om as its subject. It reads the three sounds of Om as the waking, dreaming, and deep-sleep states, and the silence that follows them as turiya, the fourth. Sage Gaudapada's Karika is the commentary traditionally read alongside it.
✦ Did you know
The Mandukya Upanishad is twelve verses long. An entire school of Vedanta is built on those twelve.
Source: Mandukya Upanishad