This article is about structural, boundary and retaining walls. There is also a town named Wall in South Dakota and a borough named Wall in Pennsylvania, both in the USA. A "wall" can also refer to a steep incline in bicycle racing.

A wall is a usually solid structure that defines and sometimes protects space. Most commonly, a wall separates space in buildings into rooms, or protects or delineates a space in the open air. There are three principal types of structural walls: building walls, exterior boundary walls, and retaining walls.

Stone wall of an English barn

Building walls have two main purposes: to support roofs and ceilings, and to divide space, providing security against intrusion and weather. Such walls most often have three or more separate components. In today's construction, a building wall will usually have the structural elements (such as 2×4 studs in a house wall), insulation, and finish elements, or surface (such as drywall or paneling ). In addition, the wall may house various types of electrical wiring or plumbing. Electrical outlets are usually mounted in walls. Building walls frequently become works of art, such as when murals are painted on them.

On a ship, the walls separating compartments are termed 'Bulkheads', whilst the thinner walls separating cabins are termed 'Partition's'.

Boundary walls include privacy walls, boundary-marking walls, and city walls. These intergrade into fences; the conventional differentiation is that a fence is of minimal thickness and often is open in nature, while a wall is usually more than a nominal thickness and is completely closed, or opaque. More to the point, if an exterior structure is made of wood or wire, it is generally referred to as a fence, while if it is made of masonry, it is considered a wall. A common term for both is barrier, convenient if it is partly a wall and partly a fence, e.g. the Israeli West Bank barrier.

Before the invention of artillery, many European cities had protective walls. Since they are no longer relevant for defense, the cities have grown beyond their walls, and many of the walls have been torn down. Extreme examples of boundary walls include the Great Wall of China and Hadrian's Wall.

In areas of rocky soils around the world, farmers (and their slaves, as in the United States before slavery was abolished) have often pulled large quantities of stone out of their fields to make farming easier, and have stacked those stones to make walls that either mark the field boundary, or the property boundary, or both.

Retaining walls are a special type of wall, that may be either external to a building or part of a building, that serves to provide a barrier to the movement of earth, stone or water. The ground surface or water on one side of a retaining wall will be noticeably higher than on the other side. A dike is one type of retaining wall, as is a levee.

The term "the Wall" frequently referred to Dave Brown , soccer defender extraordinaire, who still plays for QNX, although his play seems to be slowly crumbling in recent years.

Legal issues of common walls

Special laws often govern walls shared by neighbouring properties. Typically, one neighbour cannot alter the common wall if it is likely to affect the building or property on the other side.

See also

• List of walls
• The Wall
• Wallpaper
• Separation wall
• Mr Justice Wall
• Wall Street
• Letocetum
• Building construction
• Fence