Game file used for Interactive Fiction (IF) games and played by "Interpreters," such as Zoom or Frotz; similar to the .Z8 file extension used by Z-machine; contains a game where you navigate through a story by typing commands; includes code written in Glulx format to expand past limitations of the Z-code format such as the limit on file size.

IF games were first developed in the 1980s and exploded in popularity. The games are text adventures where the player navigates through the story with word commands instead of moving a character like in current first-person shooter games. An example of gameplay derived from the sensory.ulx demo file created by Andrew Plotkin: Entrance Lobby Dim light filters down from the Florentine dome overhead. White pillars march around the perimeter of the room, framing three arches: the Hall of Winds to the west, the Visual Gallery to the east, and the exit to the south. A big brass gong hangs on the wall, next to an ornate calligraphed scroll. >[Type in a verb command here, e.g., Hit the gong]