Minecraft wiki entry for Piston, associated text: Designs D, E, and F are straight-line versions, 1x2x2. Design D is a rising edge detector; design F, a falling edge detector; both with an output pulse of 2 ticks. Design E is a zero crossing detector, activating on both rising and falling edges. However, it produces only a single-tick pulse output. This can be moderated by adding a 2-tick repeater to the output, producing a 2-tick pulse, same as the other two.