The Gemini by Petzl is a double prusik pulley designed for building technical pulley systems. With the lower attachment loop, you can set up all kinds of pulleys. The specific shape of the side pieces allows you to tie the rope onto the hanging point with a Prusik knot. The hanging point acts as a backstop without slipping into the rope pulley. Both the side pieces and the rope pulley are made of aluminium and therefore very light!