The Quake sources as originally release under the GPL license on December 21, 1999

readme.txt

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+
+This is the complete source code for winquake, glquake, quakeworld, and 
+glquakeworld.
+
+The projects have been tested with visual C++ 6.0, but masm is also required 
+to build the assembly language files.  It is possible to change a #define and 
+build with only C code, but the software rendering versions lose almost half 
+its speed.  The OpenGL versions will not be effected very much.  The 
+gas2masm tool was created to allow us to use the same source for the dos, 
+linux, and windows versions, but I don't really recommend anyone mess 
+with the asm code.
+
+The original dos version of Quake should also be buildable from these 
+sources, but we didn't bother trying.
+
+The code is all licensed under the terms of the GPL (gnu public license).  
+You should read the entire license, but the gist of it is that you can do 
+anything you want with the code, including sell your new version.  The catch 
+is that if you distribute new binary versions, you are required to make the 
+entire source code available for free to everyone.
+
+Our previous code releases have been under licenses that preclude 
+commercial exploitation, but have no clause forcing sharing of source code.  
+There have been some unfortunate losses to the community as a result of 
+mod teams keeping their sources closed (and sometimes losing them).  If 
+you are going to publicly release modified versions of this code, you must 
+also make source code available.  I would encourage teams to even go a step 
+farther and investigate using public CVS servers for development where 
+possible.
+
+The primary intent of this release is for entertainment and educational 
+purposes, but the GPL does allow commercial exploitation if you obey the 
+full license.  If you want to do something commercial and you just can't bear 
+to have your source changes released, we could still negotiate a separate 
+license agreement (for $$$), but I would encourage you to just live with the 
+GPL.
+
+All of the Quake data files remain copyrighted and licensed under the 
+original terms, so you cannot redistribute data from the original game, but if 
+you do a true total conversion, you can create a standalone game based on 
+this code.
+
+I will see about having the license changed on the shareware episode of 
+quake to allow it to be duplicated more freely (for linux distributions, for 
+example), but I can't give a timeframe for it.  You can still download one of 
+the original quake demos and use that data with the code, but there are 
+restrictions on the redistribution of the demo data.
+
+If you never actually bought a complete version of Quake, you might want 
+to rummage around in a local software bargain bin for one of the originals, 
+or perhaps find a copy of the "Quake: the offering" boxed set with both 
+mission packs.
+
+Thanks to Dave "Zoid" Kirsh and Robert Duffy for doing the grunt work of 
+building this release.
+
+John Carmack
+Id Software
+
+