Editing Doom engine (section)

===Sidedefs===
Sidedefs are used to store wall [[Texture mapping|textures]]; these are completely separate from the floor and ceiling textures. Each sidedef can have three textures; these are called the middle, upper and lower textures. In one-sided linedefs, only the middle texture is used for the texture on the wall. In two-sided linedefs, the situation is more complex. The lower and upper textures are used to fill the gaps where adjacent sectors have different floor and ceiling heights: lower textures are used for steps, for example. The sidedefs can have a middle texture as well, although most do not; this is used to make textures hang in mid air. For example, when a transparent bar texture is seen forming a cage, this is an example of a middle texture on a two-sided linedef.