LaTeX Project Public License
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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 is also used for most third-party LaTeX packages. Software projects other than LaTeX rarely use it.
Unique features of the license
The LPPL grew from Donald Knuth's original license for TeX, which states that the source code for TeX may be used for any purpose but a system built with it can only be called 'TeX' if it strictly conforms to his canonical program. The incentive for this proviso was to ensure that documents written for TeX will be readable for the foreseeable future – and indeed, TeX and its extensions will still compile documents written from the early 1980s to produce output exactly as intended. Quoting Frank Mittelbach, the main author of the license: "LPPL attempts to preserve the fact that something like LaTeX is a language which is used for communication, that is if you write a LaTeX document you expect to be able to send it to me and to work at my end like it does at yours".
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, both in the source and also when called in some sort of interactive mode. A name change of the work is still recommended, however. This appeased the Debian community.
In order to provide project continuity in the case that the copyright holder no longer wishes to maintain the work, maintenance can be passed on to another (or from maintainer to maintainer). This can either be declared by the copyright holder or, in the event that the copyright holder is no longer able to be contacted, by the individual taking over maintenance, with a three month gap after their public intention to take over the maintenance. The modifying clause discussed above does not hold for the current maintainer of the work.
The LaTeX 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. While copying and distribution is allowed, changing the text of the LPPL is not. However, it may be used as a model for other licenses provided they do not reference the LPPL.