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I was reading through the LGPL license and found a requirement that I was previously unaware of.

Section 4 (Combined Works) states:

You may convey a Combined Work [...] if you also do each of the following:

a) Give prominent notice with each copy of the Combined Work that the Library is used in it and that the Library and its use are covered by this License.

b) Accompany the Combined Work with a copy of the GNU GPL and this license document.

c) ...

Why must the GPL also be conveyed when linking to libraries licensed under the LGPL? I fear that including both licenses in the distribution would confuse the user as to which one applies. Am I interpreting this correctly? If so, what is the reasoning behind this requirement?

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The LGPL is not a license of its own, but an extension of the GPL. It states:

This version of the GNU Lesser General Public License incorporates the terms and conditions of version 3 of the GNU General Public License, supplemented by the additional permissions listed below.

The LGPL license document only contains the extensions, so to provide the complete license you will also need to copy the vanilla GPL. It is reasonable to require a user of a LGPL library to provide the full license of the library they are using.

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  • Makes sense. So the libraries themselves should also be including both and not just a single LICENSE file containing the contents of the LGPL. – zmb Nov 12 '15 at 15:50
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    @zmb I would concatenate the two licenses into a single LICENSE file. Alternatively, the LICENSE file only mentions that the library is available under the GPL or LGPL, and refers to separate files LICENSE-GPL and LICENSE-LGPL or so. – amon Nov 12 '15 at 15:54
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The likely reason is that the LGPL is considered a less restrictive license than the GPL. In particular, it is possible to combine LGPL projects with other projects and not force those projects to be LGPL. That's a lot more difficult to do with a GPL project. Generally, if you combine a project with a GPL project, the combined project must be released as GPL.

In order to combine an LGPL project with a GPL project, the restrictions of the GPL project need to be maintained.

Taking a step back, and looking at it from the broader FSF philosophy of "Software is meant to be Free", you can see why they would have that clause within the LGPL. LGPL doesn't have as much of a "viral effect" as the GPL does, and the FSF wants GPL'd software to be preserved as being under the GPL.

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    To put it another way, the LGPL is the GPL with a linking exception. – Robert Harvey♦ Nov 12 '15 at 15:44

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