Minecraft wiki entry for Tripwire_Hook, associated text: In order to function correctly, a tripwire hook must be part of a "tripwire circuit": a straight line of blocks consisting of a block with a tripwire hook attached to it, a "tripwire line" (1 to 40 blocks of tripwire), and a second tripwire hook attached to another block, with both tripwire hooks facing the tripwire.