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Institute for Legal Questions on Free and Open Source Software

What types of licenses are there for Open Source software, and how do they differ?

The rights of use granted in the countless Open Source licenses ( only differ slightly (→ How can I use open source software? What rights of use to I acquire?), but there can be substantial differences in the required terms of the license.  Although the licensee may not request any license fees, licensees are commonly subject to other requirements.
The most significant differences have to do with the provisions of the license for development.  Whereas "Non-copyleft licenses" enable the licensee to license his modifications to the original software as either Open Source software or "proprietary" software (→ What is “proprietary software” and “closed source software"), "Copyleft licenses" require licensee to license specific developments (if they are not restricted to internal use) to anyone under the original license. This ensures that everyone who profits from open-source software will offer their developments to the developer community.
The "copyleft effect" can take different forms.  Strict copyleft licenses demand that any changes be released as open source software, whereas limited copyleft licenses only require this in certain cases.  For example, the Mozilla Public License (MPL) only requires that changes to existing files have to be released, whereas added files can be licensed in any way. The Lesser General Public License (LGPL) for program libraries only requires that changes to the library itself are subject to the LGPL. The program accessing the library can have any type of license.
With many Open Source licenses, it is quite difficult to assess when the copyleft effect starts (→ When does independently developed software have to be licensed under the GPL?).
Next FAQ: What are the most important open source licenses, and what type of license are they?

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