Lewmar 50mm Synchro Block - Single. Using a combination of scientifically optimized block geometry a Free-Spin bearing and a self-aligning head to reduce friction and increase efficiency by up to 40percent over budget blocks on mainsheet systems.Each component is perfectly synchronized with the movement of the rope providing you with an easier smoother transfer of power from deck to sail and less wear on your rope. Package Contents: Lewmar 50mm Synchro Block Lewmar 29925001BK: 50mm Synchro Block - Single