The four nails or “four nails that secure the life-force of the practice” (Tib. སྲོག་སྡོམ་གཟེར་བཞི་, sok dom zer shyi, Wyl. srog sdom gzer bzhi), an important aspect of kyerim practice, are:
- The nail of all appearance as the deity (nangwa lha yi zer) or the nail of concentration (samadhi) on the deity (ting nge dzin lha’i zer) or (kyerim lha'i zer)
- The nail of all sound as the mantra (dra drak ngak kyi zer) or the nail of the essence mantra (nyingpo ngak kyi zer)
- The nail of the activity of emanation and absorption (tro du trinlé kyi zer)
- The nail of the unchanging wisdom mind (gongpa mingyurwé zer)
According to Nyoshul Khen Rinpoche, these four points were first taught as a special mengak by Guru Padmasambhava in the Kagyé Deshek Düpa.
- His Holiness Dilgo Khyentse Rinpoche, Pure Appearance—Development and Completion Stages in the Vajrayana Practice (Halifax: Vajravairochana Translation Committee, 2002), '5. The Four Binding Forces'. restricted publication
- Patrul Rinpoche in Deity, Mantra and Wisdom: Development Stage Meditation in Tibetan Buddhist Tantra, translated by the Dharmachakra Translation Committee (Ithaca: Snow Lion, 2007), Text Three, The Melody of Brahma Reveling in the Three Realms: Key Points for Meditating on the Four Stakes That Bind the Life-Force.
- Padmasambhava and Jamgön Kongtrül, The Light of Wisdom, Vol II, translated by Erik Pema Kunsang (Boudhanath, Hong Kong & Esby: Rangjung Yeshe, 1998), pages 108-117.