Redstone comparator

A redstone comparator can be broken instantly using any tool, or without a tool, and drops itself as an item. To remove a redstone comparator, use the attack control.
A redstone comparator will also be removed and drop itself as an item:

• if its attachment block is moved, removed, or destroyed
• if water flows into its space
• if a piston tries to push it or moves a block into its space

If lava flows into a redstone comparator's space, the redstone comparator will be destroyed without dropping itself as an item.
A redstone comparator can be broken instantly using any tool, or without a tool, and drops itself as an item. To remove a redstone comparator, use the attack control.
A redstone comparator will also be removed and drop itself as an item:

• if its attachment block is moved, removed, or destroyed
• if water flows into its space
• if a piston tries to push it or moves a block into its space

If lava flows into a redstone comparator's space, the redstone comparator will be destroyed without dropping itself as an item.
A redstone comparator can be placed on the top of any opaque block with a solid full-height top surface (including upside-down slabs and upside-down stairs), as well as on hoppers. To place a redstone comparator, use its item on the top of a block.
The redstone comparator has a front and a back — the arrow on the top of the comparator points to the front. When placed, the comparator will face away from the player. The comparator has two miniature redstone torches at the back and one at the front. The back torches turn on when the comparator's output is greater than zero (the arrow on top also turns red). The front torch has two states which can be toggled by use on the comparator: lowered and non-powered (indicating the comparator is in "comparison mode"), and elevated and powered (indicating the comparator is in "subtraction mode").
The redstone comparator can take a signal strength input from its rear as well as from both sides. Side inputs are only accepted from redstone dust, redstone repeaters, and other comparators. The redstone comparator's front is its output.
It takes one redstone tick (two game ticks) for signals to move through a redstone comparator, either from the rear or from the sides. This applies to changing signal strengths as well as simply to turning on and off. Redstone comparators usually will not respond to 1-tick fluctuations of power or signal strength.
The redstone comparator has four functions: maintain signal strength, compare signal strength, subtract signal strength, and measure certain block states (primarily the fullness of containers).

Maintain signal strength

A redstone comparator with no powered side inputs will simply output the same signal strength as its rear input.

Compare signal strength

Comparators in comparison mode.

A redstone comparator in comparison mode (front torch lowered and non-powered) will compare its rear input to its two side inputs. If either side input is greater than the rear input, the comparator output turns off. If neither side input is greater than the rear input, the comparator simply outputs the same signal strength as its rear input.