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I know that LaTeX is free. But is it open source? Is it possible to obtain the source code for LaTeX? From what I understand TeX is open source. Are the macros that make up LaTeX open source?

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    "LaTeX" is a set of macros that builds on the so-called TeX primitives; nothing there to be compiled. Are you asking if pdfTeX, XeTeX, and LuaTeX -- three main TeX "engines" these days -- are open source?
    – Mico
    Oct 11, 2015 at 21:46
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    @Thomas: The core of LaTeX is fully defined in latex.ltx, readable (and sometimes understandable) and the documented in source2e.pdf. Since LaTeX is build on top of TeX it's Open Source. This might not be true for any package, if there's a special license.
    – user31729
    Oct 11, 2015 at 21:50
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    @ChristianHupfer :-) Oct 11, 2015 at 22:54
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    Wikipedia exists: en.wikipedia.org/wiki/LaTeX_Project_Public_License Oct 12, 2015 at 5:10
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    No discussion on this is complete without a link to Frank's TUGboat: tug.org/TUGboat/tb32-1/tb100mitt.pdf
    – Joseph Wright
    Oct 12, 2015 at 12:57

3 Answers 3

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I would recommend you read up on The LaTeX project public license (LPPL), which may be accessed from http://www.latex-project.org/lppl/. Here's an excerpt from that site:

The LaTeX project public license is a free software license. The most recent version of the LPPL is version 1.3c. The latest version, in plain text, can always be found at https://latex-project.org/lppl.txt. There is also a LaTeX version of the license, which authors of LPPL software are encouraged to include in their manuals.

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    @Thomas - The LPPL does have some restrictions. For instance, "You may distribute a complete, unmodified copy of the Work as you received it. Distribution of only part of the Work is considered modification of the Work, and no right to distribute such a Derived Work may be assumed under the terms of this clause." And, "If you are not the Current Maintainer of the Work, you may modify your copy of the Work, thus creating a Derived Work based on the Work, and compile this Derived Work, thus creating a Compiled Work based on the Derived Work." Key take-away: You can modify copies.
    – Mico
    Oct 11, 2015 at 22:00
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    @Thomas LPPL is considered an open source licence by Debian (who are very strict about these things) (most of the changes in the current version 1,3 of the LPPL were specifically to remove some concerns raised on Debian-legal) Oct 11, 2015 at 22:10
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    The Wikipedia article has a good discussion of the renaming/modification notice requirement and why this is accepted as free software by Debian, FSF, and OSI, but not "copyleft" per GPL compatibility.
    – hardmath
    Oct 12, 2015 at 2:49
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    @Thomas: Both OSI and FSF have accepted it as "open source" and "free software". Oct 12, 2015 at 5:09
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    @cfr, you can distribute modified versions. Just not under the original name. This is explicitly allowed by OSI (in part to accomodate TeX, I presume).
    – vonbrand
    Oct 12, 2015 at 12:37
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To expand on @Mico's answer: TeX and (base) LaTeX are just a (small) part of the hundreds of macro packages and programs that make up a LaTeX installation, like MikTeX or TeXlive. Each piece is under it's own license, mostly LPPL, but there is stuff under other licenses. The vast majority is under free/open licenses, but you'd have to check each piece to know for sure (or trust, e.g., TeXlive or your Linux distribution to have done due diligence).

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    It's worth adding that vitually all packages distributed via TeXLive satisfy either the LPPL or some other "free/open" license, such as the MIT style license. Some interesting packages, however, do not meet these licensing criteria; they may be found on the CTAN even though they (by design) cannot be distributed via TeXLive.
    – Mico
    Oct 12, 2015 at 13:45
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About the open source, I think we can listen to the FSF (Free Software Fundation) and licenses and comment about them:

LaTeX Project Public License 1.3a (#LPPL-1.3a)

We have not written a full analysis of this license, but it is a free software license, with less stringent requirements on distribution than LPPL 1.2 (described next). It is still incompatible with the GPL because some modified versions must include a copy of or pointer to an unmodified version.

They considered it as as a free licence with incompatibility with some other free licenses that are more restrictive.

Furthermore, as said before, some parts use in some LaTeX distribution are under other licenses as GPL/LGPL for example.

Edit:

Thanks to @JosephWright I add the open source initiave point of vue:

LPPL is one license that satisfies the requirements to be defined as Open Source by the open source initiative. You can access to every of those licenses in the link given.

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    Note that in this context 'free' and 'open' tend to be separate terms: the FSF are interested in freedom in an intellectual sense, which is not the same as the way the Open Source Initiative think about things. (In particular, I'd note that the FSF place a strong emphasis on freedom for the programmer/developer.)
    – Joseph Wright
    Oct 12, 2015 at 12:55
  • @JosephWright I've modified my answer to add the OSI part thanks for pointing that ;-) Oct 12, 2015 at 13:03

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