Minecraft wiki entry for Piston, associated text: Pronounced "ex-nor", this device activates when both inputs are equal. A useful attribute is that a XOR or XNOR gate always changes its output when one of its inputs changes, allowing for 2 switches to be combined to open or close a door, or activate another device. This design is significantly smaller than the redstone-only equivalent and slightly faster. Adding a NOT gate to the end or to either input produces an XOR gate, which activates when the inputs are different.