A ladder is a vertical set of steps used to reach high areas. There are two types of ladders: 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 a buildings walls.
Rigid ladders are available in many forms, such as: fixed ladder (two stiles joined by several rungs; no moving parts), extension ladder (fixed ladder divided into two or more sliding lengths for more convenient storage), step ladder (hinged in the middle to form an inverted V, with stays to keep the two halves at a fixed angle), and the roof ladder (rigid ladder with large hook at the top to grip the ridge of a pitched roof). ALthough there are still other types of ladders, these are some popular ones.
For safety, a rigid ladder should be leaned at an angle of about fifteen degrees to the height. 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.