Minecraft wiki entry for Redstone_components, associated text: To place tripwire, right-click on an adjacent block with a string. Tripwire can be placed on the ground or in the air, and forms a valid tripwire line only if all the tripwire is of the same type. Tripwire is considered on the ground if placed on any opaque block, or on a block of redstone, a hopper, an upside-down slab, or an upside-down stairs. Tripwire is considered in the air if placed on or above any other block. Tripwire on the ground has a short hitbox (1/8 block tall), while tripwire in the air has a taller hitbox (1/2 block tall).