- A typical option tree has the following subdirectories:
  cfg
    Holds the configuration information.
    Always includes the command dictionary `commands.cnf`.
  exp
    Holds explain files for every command.
  mcr
    Holds macros.
  prim
    Holds primitives.
  test
    Holds test macros and JUnit tests.
- Additional directories found in some option trees:
  dat
    Holds any required data files.
  docs
    Option tree documentation.
  host
    Holds X-Midas host primitives used via XBC.
  inc
    Holds any include files or Java interface definitions.
  intr
    Holds intrinsic commands.
  lib
    Holds any libraries.
    Option trees with many libraries will split them up into `lib` (general purpose), `libg` (graphical), `libm` (math), `libxm` (X-Midas compatibility), and `net` (networking).
- The root directory of an option tree has the following special files:
  version.txt
    Version information about the option tree.
  updates.txt
    Lists any changes made to the option tree after its release.