Three Kinds of Free Software (GPL, LGPL, X11)

Here is how I understand the three categories of releasing free software:

If someone downloads some source code licensed under the GNU General Public License (GPL), compiles it, and distributes those binaries, they are required to distribute the corresponding source code as well. If they made modifications to the source, they must include their modified versions. If they integrate any of the code into some bigger application, the whole thing must be licensed under the GPL - or not distributed at all. Private use in any way is okay, though. This ensures that no middle men can strip out the freedom - all users of the software will have freedom for all versions.

If someone downloads some source code licensed under the GNU Lesser General Public License (LGPL), compiles it, and distributes those binaries, they are required to distribute the corresponding source code as well. If they made modifications to the source, they must include their modified versions too. But if they integrate any of the code into some bigger application, the part that is LGPL must continue to be licensed under the LGPL - but the proprietary parts can remain proprietary.

If someone downloads some source code licensed under the X11 license, compiles it, and distributes those binaries, they are not required to distribute the corresponding source code as well. Not even if they made modifications to the source, such as integrating it into some big application. This respects some users’ freedom, but middle men will make proprietary versions that strip away the freedom and add new features, making it less than ideal.

For example, say I wrote some python scripts and Pyrus integrated them into FontLab 6. With X11, they could improve them with some great new but incompatible features and release them under proprietary terms. Since the code of the python scripts would be visible, yet I’d have no freedom to do anything with the code I could see - and be unable to integrate their improvements with my originals - I’d find this very frustrating. With GPL, they wouldn’t integrate them (since that would mean GPL’ing FontLab…) but people would be free to integrate them on their own (ie, privately). With LGPL, they could integrate them, but I wouldn’t get caught in that frustrating situation.

(There are other licenses that achieve the same ends, but these are the most popular licenses of their kind.)

Creative Commons License
The Three Kinds of Free Software (GPL, LGPL, X11) by David Crossland, except the quotations and unless otherwise expressly stated, is licensed under a Creative Commons Attribution-Share Alike 3.0 Unported License.

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