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The LaTeX Project Public License (LPPL) is a software license originally written for the LaTeX system. Software distributed under the terms of the LPPL can be regarded as free software, however it is not copylefted.
Besides the LaTeX base system, the LPPL has been used for many LaTeX packages, too. Outside the LaTeX world it has no significance however.
The filename clause
The most unusual part of the LPPL – and equally the most controversial – used to be the 'filename clause': You must not distribute the modified file with the filename of the original file. This feature made some people deny that the LPPL is a free software license. In particular the Debian Linux community considered excluding LaTeX from its core distribution because of this.
However, version 1.3 of the LPPL has weakened this restriction. Now it is only necessary that modified components identify themselves "clearly and unambiguously" as modified versions, also when called in some sort of interactive mode. This appeased the Debian community.
Copyright holders
The LaTeX 3 Project holds the copyright for the text of the LPPL, but it does not necessarily hold the copyright for a work released under the LPPL. The author of the work holds copyright for the work, and is responsible for enforcing any violations of the license (or not).
Unlike the works released under the LPPL, the LPPL itself is not freely modifiable, since it is a software license. While copying and distribution is allowed, changing the text of the LPPL is not.
External links
"LaTeX Project Public License"