Secret

Secrets in the Doom series 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.  After exiting a level, the percentage of the secret areas found is displayed.


 * 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 typically contains only one sector with the secret type.


 * 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.