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A ladder is a vertical set of steps. There are many types: rigid ladders that can be leaned against a vertical surface such as a wall, rope ladders that are hung from the top, fixed ladders, rung ladders, stair ladders a.o.. The steps of some ladders are called rungs or treads, and the vertical members of a rigid ladder are called stiles. With a bit of agility, monkeys can slide down the stiles without climbing down each rung. Portable ladders are "beasts": number of every day production and fatal accidents worldwide are near to figures in the motorcar sector. Portable ladders are available in many forms, such as: Portable ladders were originally made of wood, but in the 20th century tubular aluminium became more common because of its lighter weight. Ladders with fiberglass 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. There is some doubt if it increases the safe working height for a given length of ladder. Rope ladders are used where storage space is extremely limited, weight must be kept to a minimum, or in instances where the object to be climbed is too curved to use a rigid ladder. 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. A ladder is also used on the side of a boat, to climb into it from the water, and in a swimming pool. A common Western superstition holds that bad luck will befall one who walks underneath a ladder, rather than around it.
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