Richard's recent comments regarding the MMIX license (which is derived from the licenses for other (C)WEB programs) reminded me of a different issue. tex.web says this (word-wrapped): % This program is copyright (C) 1982 by D. E. Knuth; all rights are % reserved. Copying of this file is authorized only if (1) you are % D. E. Knuth, or if (2) you make absolutely no changes to your % copy. (The WEB system provides for alterations via an auxiliary % file; the master file should stay intact.) See Appendix H of the % WEB manual for hints on how to install this program. And see % Appendix A of the TRIP manual for details about how to validate it. Note that there is no permission to distribute or make derivative works. There is something approaching a public domain dedication published here: <https://ntg.nl/maps/pdf/5_34.pdf> I do not know a more authoritative source for this statement. In part it reads: “My work on developing TeX, METAFONT, and Computer Modern has come to an end. I will make no further changes except to correct extremely serious bugs. “I have put these systems into the public domain so that people everywhere can use the ideas freely if they wish. […] “At the time of my death, it is my intention that the then-current versions of TeX and METAFONT be forever left unchanged, except that the final version numbers to be reported in the “banner” lines of the programs should become [$\pi$ and $e$]. […] “As stated on the copyright pages of Volumes B, D, and E, anybody can make use of my programs in whatever way they wish, as long as they do not use the names TeX, METAFONT, or Computer Modern. In particular, any person or group who wants to produce a program superior to mine is free to do so. However, nobody is allowed to call a system TeX or METAFONT unless that system conforms 100% to my own programs, as I have specified in the manuals for the TRIP and TRAP tests. […]” I'm not sure if this can be considered a valid public domain dedication, considering the “be forever left unchanged” part. The situation is worse with pdfTeX. It claims to be licensed under the GPL, but it is derived from code whose license appears GPL-incompatible. See below for the pdfTeX license text (again word wrapping changed for presentation purposes). % pdfTeX is free software; you can redistribute it and/or modify it % under the terms of the GNU General Public License as published by % the Free Software Foundation; either version 2 of the License, or % (at your option) any later version. % pdfTeX is distributed in the hope that it will be useful, but % WITHOUT ANY WARRANTY; without even the implied warranty of % MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU % General Public License for more details. % You should have received a copy of the GNU General Public License % along with this program. If not, see % <http://www.gnu.org/licenses/>;. % e-TeX is copyright (C) 1999-2014 by P. Breitenlohner (1994,98 by the % NTS team); all rights are reserved. Copying of this file is % authorized only if (1) you are P. Breitenlohner, or if (2) you make % absolutely no changes to your copy. (Programs such as TIE allow the % application of several change files to tex.web; the master files % tex.web and etex.ch should stay intact.) % See etex_gen.tex for hints on how to install this program. % And see etripman.tex for details about how to validate it. % This program is directly derived from Donald E. Knuth's TeX; […] I don't think we have public domain dedication for the e-TeX changes, like the limited one I cited for TeX proper. The WEB and CWEB systems allow patching of source files without changing them. As a result, the restriction on source file changes was not considered particularly onerous at the time the DFSG was written. However, I doubt it is compatible with the letter of the GPL. In any case, (C)WEB sources which carry this restriction usually lack explicit permission to distribute and make derivative works. Florian