Minecraft wiki entry for Clock, associated text: How it works: When the lever turns on (t = 0 redstone ticks), the sticky piston begins to extend. At t=1, the torch turns off, but the left repeater stays powered for 1 more tick. At t=1.5, the piston finishes extending and the moved block gets powered by the left repeater. At t=2, the left repeater turns off. At t=2.5, the right repeater begins to output the power passed to it by the block. From here on, it just continues as a 1-clock until the lever is turned off, breaking the loop.