Any area of a map directly behind a one-sided linedef is void space (which cannot be occupied except by using the idclip cheat code). A two-sided linedef is needed (to separate the two sectors) any time there is a change in (1) the height or texture of the floor or ceiling, (2) the light level, or (3) the sector tag or type.
There are three ways to trigger an action:
- "use" (or "push") (with the space bar, by default)
- shoot (with an impact weapon)
Which of these applies depends on the linedef type number, which also specifies what particular action will occur.
The specified action usually will take place in the sector or sectors that have the same tag number as the linedef. Exceptions to this include local doors (which act on the sector on the other side of the line), special effects that apply to the linedef itself, and the exit-level actions.
Most actions can be specified as either once-only or repeating (by using different linedef types).
Each linedef is 14 bytes long.
Linedefs contain a two-byte (16 bit) field reserved for various flags. Flags are as follows:
|0||0x0001||blocks players and monsters|
|3||0x0008||upper texture is unpegged|
|4||0x0010||lower texture is unpegged|
|5||0x0020||secret (shows as one-sided on automap), and monsters cannot open if it is a door (type 1)|
|7||0x0080||never shows on automap|
|8||0x0100||always shows on automap|
|9||0x0200||can be activated more than once||X|
|10-12||0x0400||activated when used by player||X|
|10-12||0x0800||activated when crossed by monster||X|
|10-12||0x0C00||activated when hit by projectile||X|
|10-12||0x1000||activated when bumped player||X|
|10-12||0x1400||activated when crossed by projectile||X|
|10-12||0x1800||activated when used by player (with pass through)||X|
|13||0x2000||line can be activated by players and monsters||X|
|15||0x8000||blocks everything (includes gunshots and missiles)||X|