Linedefs are what make up the 'shape' (for lack of a better word) of a map. Every linedef is between two vertices and contains one or two sidedefs (which contain wall texture data). There are two major purposes of linedefs. The first is to divide the map into sectors, and the second is to trigger action specials. There are three ways to trigger an action: 1. * walkover 2. * "use" (or "push") (with the space bar, by default) 3. * shoot (with an impact weapon) Which of these applies depends on the linedef type number, which also specifies what particular action will occur.