In sailing, a block is a single or multiple pulley. One or a number of sheaves are enclosed in an assembly between cheeks or chocks. In use, a block is fixed to the end of a line, to a spar, or to a surface. A line (rope) is reeved through the sheaves, and maybe through one or more matching blocks at some far end, to make up a tackle. A pulley is a wheel on an axle or shaft that is designed to support movement and change of direction of a taut cable, or transfer of power between the shaft and cable.