Lever

A lever is a type of switch. Levers - like the button, pressure plate, and redstone torch - can be connected with redstone wire to be operated far away from the target block. Levers can be placed on any opaque block.

Crafting
Levers are made from a stick and a piece of cobblestone, and can be placed on walls, floors, and ceilings (as of Snapshot 12w24a).

Power
Levers differ from buttons in that the charge they provide is stable - once clicked a lever will remain on or off until clicked again. Clicking a lever also causes a visual change as the shaft of the device moves from one side to the other. Their stable nature makes levers well suited to controlling devices that need to remain in one of two states for an indefinite period, such as minecart tracks or locking mechanisms in advanced circuits.

When placed on the side of blocks, up is off and down is on. On the top or bottom of blocks, on is south or west, off is north or east. Mobs cannot turn on a lever, or turn it off.

Trivia

 * The lever, along with wall mounted torches, flowing lava and water, sloped rails, tripwire hook, Spider's legs and Snow Golem's arms are the only surfaces in Minecraft that are not either parallel or perpendicular to the ground.
 * The rotation of levers placed on the top of blocks is decided randomly and not influenced by the rotation of the player.
 * Anything powered by a single redstone torch, such as a powered rail block, can be replaced with a lever. This is useful if you are low on redstone. This also allows turning the current off if need be.
 * Levers cannot be placed on snow or ice.

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