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?
[Tex/LaTex] Is LaTeX open source
latex-projectlicensing
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With respect to the first part of the question, in words of Leslie Lamport:
In the early 80s, I was planning to write the Great American Concurrency Book. I was a TeX user, so I would need a set of macros. I thought that, with a little extra effort, I could make my macros usable by others. Don Knuth had begun issuing early releases of the current version of TeX, and I figured I could write what would become its standard macro package. That was the beginning of LaTeX. I was planning to write a user manual, but it never occurred to me that anyone would actually pay money for it.
In 1983, Peter Gordon, an Addison-Wesley editor, and his colleagues visited me at SRI. Here is his account of what happened. Our primary mission was to gather information for Addison-Wesley "to publish a computer-based document processing system specifically designed for scientists and engineers, in both academic and professional environments." This system was to be part of a series of related products (software, manuals, books) and services (database, production). (La)TeX was a candidate to be at the core of that system. (I am quoting from the original business plan.) Fortunately, I did not listen to your doubt that anyone would buy the LaTeX manual, because more than a few hundred thousand people actually did. The exact number, of course, cannot accurately be determined, inasmuch as many people (not all friends and relatives) bought the book more than once, so heavily was it used.
With respect the second part, LaTeX is not the only system (macro package) based in TeX. Mainly you must be aware of the existence of ConTeXt, but see https://www.ctan.org/tex-archive/macros.
Citing the CTAN LaTeX entry:
LaTeX is a widely-used macro package for TeX, providing many basic document formating commands extended by a wide range of packages. It is a development of Leslie Lamport's LaTeX 2.09, and superseded the older system in June 1994. The basic distribution is catalogued separately, at latex-base; apart from a large set of contributed packages and third-party documentation (elsewhere on the archive), the distribution includes:
– a bunch of required packages, which LaTeX authors are “entitled to assume” will be present on any system running LaTeX; and – a minimal set of documentation detailing differences from the ‘old’ version of LaTeX in the areas of user commands, font selection and control, class and package writing, font encodings, configuration options and modification of LaTeX.
It's published under the LaTeX Project License 1.3 and is considered to be free and changeable. (See LaTeX Project too)
It was developed initially by Leslie Lamport to ease the usage of TeX
(which was designed by Donald E. Knuth also named The Grand Wizard (not to be confused with the name from the infamous and dreadful KKK) in the late 70s/early 80s)
Although there are commercial TeX versions which have to be paid for the majority of users download LaTeX (and XeLaTeX/LuaLaTeX as well as TeX) either as the TeXLive or MikTeX distributions or directly from the CTAN server.
A list of current maintainers of LaTeX2e
and LaTeX 3
core can be found here: LaTeX 3.
A lot of other people design(ed) additions as style oder class packages, all based on the core -- everybody is allowed to use LaTeX and to provide own additions and formats -- that's one of the most thrilling (and sometimes difficult) features of this 'programming' language.
LaTeX users and developers meet in conferences and are organized in global communities such as TUG (TeX User Group), Dante (a German/Austrian TeX user group) and several other local/national groups as well as online like here on TeX.SX, goLaTeX, LaTeX community and some others.
Best Answer
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: