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The Internet Compass

Open Source

Copyleft License

A copyleft license is an open-source license that grants broad rights to use, modify and distribute software on the condition that any distributed derivative work is released under the same license terms — most notably the GNU General Public License (GPL) family.

"Strong" copyleft (the GPL) requires that any software linking to or incorporating the covered code, and distributed to others, also be licensed under the GPL. "Weak" copyleft (like the LGPL or Mozilla Public License) narrows that requirement to modifications of the covered code itself, allowing it to be linked into proprietary software without "infecting" the whole project.

Companies building commercial products routinely maintain license-compliance policies specifically to avoid unintentionally incorporating strong-copyleft code into proprietary codebases, since the obligation to release derivative source code can apply even without anyone realising a dependency carried it.