Doom 1.9

Doom version 1.9 was the last version of Doom released by id. Despite the version numbering, three distinct versions of "Doom 1.9" exist.

Original Doom 1.9
The first Doom 1.9 was used for Doom shareware, Registered Doom and Doom II. All three distributions use the exact same executable file (the binaries are identical). In Shareware and Registered Doom, this is named doom.exe. In Doom II, this is named doom2.exe.

Ultimate Doom
The Ultimate Doom executable is different from the original, but is still described as "version 1.9" on startup. The main differences between this and the original Doom 1.9 are changes made to support Ultimate Doom. The executable can be used to play with IWADs other than the Ultimate Doom executable, but doing so under Doom II will cause the game to crash when opening the help screen.

The major difference in behavior of the engine is that Tag 666 behaves differently, breaking some WADs such as Doomsday of UAC. Another difference is in the physics behavior of Lost Souls. This was to fix a bug in the collision response when a lost soul hits the ground; however, the introduction of this fix broke demo compatibility with some demos.

Final Doom
The Final Doom executable is derived from the Ultimate Doom executable. Again, this is also described as "version 1.9" on startup. The same executable is used for both the TNT: Evilution and The Plutonia Experiment; the game setting is selected depending on the IWAD name (either tnt.wad or plutonia.wad).

The engine was modified for Final Doom in order to support the new level names and intermission text for the new IWADs. However, the behavior of teleporters was also changed in this new version. The "Z" position of objects being teleported is set to the floor in previous versions, but is not in Final Doom. As a result, teleported monsters and players may fall to the ground if they teleport from a higher position. This can result in demo incompatibilities between this version and older versions.

As the Final Doom executable was derived from the Ultimate Doom source, it also includes the fixed lost soul behavior.