Three Refuge formulations

In Tibetan Buddhism there are three Refuge formulations, the Outer, Inner and Secret forms of the Three Jewels. The 'Outer form is the 'Triple Gem', (Sanskrit:Triratna), the 'Inner' is the Three Roots and the 'Secret form is the 'Three Bodies' or Trikaya of a Buddha. These alternative Refuge formulations are employed by those undertaking Deity Yoga and other tantric practices within the Tibetan Buddhist Vajrayana tradition as a means of recognizing Buddha-nature.