Secret

Secrets can mean several things:


 * Areas of a level that are hidden (to some extent). Usually, there is some clue about a secret area, such as an off-color wall that is actually a door.  By convention, most levels are designed so that it is not necessary to find any of the secret areas to complete the level.


 * A special sector type (9), used to calculate the secrets-found percentage displayed at the end of each level. The number of type 9 sectors in the level is the total number of secrets.  When a player first enters a type 9 sector, the number of secrets found is incremented by one.  A secret (or "hidden") area may be any number of sectors, but for proper operation of the stats will normally contain only one sector with type 9.  Some source ports use the type 9 sector to trigger an announcement such as " A SECRET HAS BEEN REVEALED " and to play a sound, but the original game does not give any indication until the intermission screen.


 * A linedef flag bit (5). This makes a two-sided linedef appear on the automap in red instead of yellow, that is, just like a single-sided linedef.  This is used to hide entrances to secret areas on the automap, but has nothing to do with the calculation of the secrets percentage displayed at the end of each level.