Four classes of tantra
One inner class of tantra, which is called
- Anuttarayoga Tantra, or Highest Yoga Tantra.
The latter is divided into three:
- father tantras, such as the Guhyasamaja, the King of Tantras;
- mother tantras, such as Heruka Chakrasamvara; and
- non-dual tantras, such as Kalachakra.
- The Kalachakra Tantra can be designated either as a mother tantra or non-dual tantra. See Kalachakra Tantra, Rite of Initiation by the Dalai Lama and Jeffrey Hopkins (London: Wisdom, 1985) pages 165-6.