The Rig Veda is the book of Mantra. It contains the oldest form of all the Sanskrit mantras. It is built around a science of sound which comprehends the meaning and power of each letter. Most aspects of Vedic science like the practice of yoga, meditation, mantra and Ayurveda can be found in the Rig Veda and still use many terms that come from it.

How where the sounds of the letters established?
How was the geometry (for example :why does the letter 'A' look like an A)of each letter established?