Wall textures are graphics built up from several lumps in the WAD files and used to cover wall surfaces. Complete details of the data structures are given in the Unofficial Doom Specs. Each wall consists of a linedef that references one or two sidedefs (right and maybe left). Each sidedef has three texture names (lower, middle, and upper) with up to eight characters each. (The name "-" means no texture: either not applicable or totally transparent.) Wall textures must be exactly 128 units high, or the tutti-frutti effect will occur. There is no maximum width.