Four Nails. These are the four nails which control life-breath {srog sdom gzer bzhi} according to Yoga Tantra, namely, the nail of unchanging intention {mi 'gyur dgongs pa'i gzer} the nail of contemplation on the deity {ting 'dzin lha'i gzer} the nail of mantra which is the nucleus {snying po sngags kyi gzer} and the nail of activity which is the emanation and absorption of light rays {phro 'du phrin las kyi gzer}