Four classes of tantra 四部密續
And 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 Tantra, the King of Tantras;
- Mother Tantras, such as the Chakrasamvara Tantra; and
- Non-dual Tantras, such as Kalachakra.
- Glenn H. Mullin, The Dalai Lamas on Tantra (Snow Lion Publications, 2006)
- 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.