This article describes the directory hierarchy as it appears to the engine at runtime.

bin/
data/
  <game-name>/
    auto/  # Special auto-load folder
    detailtextures/
    flats/
    lightmaps/
    models/
    music/
    sfx/
    textures/
    patches/
defs/
  <game-name>/
    auto/ # Special auto-load folder
run/  # set with -userdir

Most of this structure is configurable with [[command_line_option]]s (such as {{arg|texdir).}}