A Line Type defines the behavior of a map line, i.e., a wall in the map.

Line behaviors are implemented by various XG classes that define what the line does. The class's behavior is carried out when the line is activated, deactivated (or both) or when some other triggering condition occurs. Some examples of line classes include Move Plane, End Level, and Change Wall Texture.

Line types are a fundamental building block of XG. Please see the XG reference guide for more information about how to use line types.

An XG Line Type definition begins like this:

Line Type {
  ID = 5000;  # A unique ID number.

This unique ID number is required for each line type. It is the actual type number that will be used in a level editor, instead of the line types defined by the original game.

An XG line is a normal Doom line whose type is set to an XG line type. XG lines can either be active or inactive. It is possible to disable an XG line, which means the line will be skipped in all event processing and the line's timer will not be incremented.