Ajna Chakra is situated within the Sushumna Nadi and its corresponding centre in the physical body is at the space between the two eye-brows. This is known as Trikuti.
Ha - Ksha
The third eye has two sanskrit letters: Ha and Ksha, Ha the aspiration and Ksha = Ka the last letter of the heart and Sha the first letter of the root chakra, body and mind duality. Like the Egyptian Falcon (Ha) and Serpent (Ksha) headdress. In the world but not of it. http://www.givnology.com/chakras.htm