Anahata is described in the Sat-Cakra-Nirupana as a lotus of twelve petals at the heart, bearing the letters kam through tham. Within it is a hexagram of two interlaced triangles, smoke-coloured, the region of air, with the seed syllable YAM seated on a black antelope; the presiding deity is Isha with the shakti Kakini, and within it is also the bana linga and, in the associated literature, the Ananda-kanda with the wish-fulfilling tree. The name means unstruck, from anahata nada, the sound that arises without two things striking together, the unstruck sound heard in deep meditation and contrasted with all ordinary sound, which is ahata, struck. That distinction is a genuine and important one in Indian musical and meditative theory and is much older than this text.
Tibetan Vajrayana completion stage practice gives the heart region a significance of its own, independent of the Hindu scheme recorded here: in traditions descending from the Six Yogas of Naropa and related Kalachakra teachings, the heart centre, rather than the crown, is generally treated as the seat of the subtlest level of mind, the one said to remain active through the process of dying, which places the emphasis of that tradition's practice differently from the Hindu ascent toward Sahasrara. The two traditions are independent developments rather than variants of a single source, and the atlas records them separately for that reason.
Sources The Serpent PowerSir John Woodroffe (Arthur Avalon) with The Origins of Yoga and Tantra: Indic Religions to the Thirteenth CenturyGeoffrey Samuel