Wall

A wall is any vertical surface. It is represented as follows:
 * Two vertices are defined for the endpoints.
 * A linedef specifies the "from" and "to" vertices, and one or two sidedefs.
 * Each sidedef names the texture(s) displayed on one side of the wall and the sector that side faces.
 * The sector defines the floor and ceiling height (bottom and top of the wall).

A linedef, and thus a wall, can be one-sided or two-sided. One-sided walls display a single texture, and have no play area behind them. Two-sided walls can display a lower, middle, and upper texture (any of which may be empty) to help create windows and the like. (An actual window will consist of a pair of parallel, two-sided lindefs.) The "middle" is determined by the floor and ceiling of the sector on the other side of the wall.