Skull damarus such as the one pictured here is a common instrument for practitioners of Vajrayana. The two drums connected and used as a single instrument represent the inseparability of emptiness and form. One side of the drum cannot be struck without the other being struck as well, when the instrument is being played properly, which illustrates in the realm of physical form as well as that of energy, or sound, the most profound essence of Tantric teachings: non-duality. Also, the use of bone implements in Vajrayana symbolises working directly with the non-dual state.