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A ladder is a vertical set of steps. There are two types: rigid ladders that can be leaned against a vertical surface such as a wall, and rope ladders that are hung from the top. The steps of a ladder are called rungs, and the vertical members of a rigid ladder are called stiles. Rigid ladders are usually portable, but some types are permanently fixed to buildings.

Rigid ladders are available in many forms, such as:

Rigid ladders were originally made of wood, but in the 20th century tubular aluminium became more common because of its lighter weight. Ladders with fibreglass stiles are used for working on or near overhead electrical wires, because fibreglass is an electrical insulator.

For safety, a rigid ladder should be leaned at an angle of about fifteen degrees to the vertical. In other words, the distance from the foot of the ladder to the wall should be about one quarter of the height of the top of the ladder. At steeper angles, the ladder is at risk of toppling backwards when the climber leans away from it. At shallower angles, the ladder may lose its grip on the ground. Ladder stabilisers are available that increase the ladder's grip on the ground.

A ladder standoff, or stay, is a device fitted to the top of a ladder to hold it away from the wall. This enables the ladder to clear overhanging obstacles such as the eaves of a roof, and increases the safe working height for a given length of ladder.

Rope ladders are used where storage space is extremely limited, or weight must be kept to a minimum. They may have rigid or flexible rungs. Climbing a rope ladder requires more skill than climbing a rigid ladder, because the ladder tends to swing like a pendulum.

See also stairway.

The is also a programming language named Ladder