Overview

The GNU Lesser General Public License (LGPL) is a free software license created by the Free Software Foundation for software libraries and other modular components. It is an open source licence intended to balance two goals: to ensure that improvements to the library remain freely available, and to allow the library to be used by a wider range of programs, including proprietary ones, without forcing the entire combined work to adopt strong copyleft terms.

Key characteristics

The LGPL is commonly described as a "weak" or "limited" copyleft. Its principal features include:

  • Scope of copyleft: The copyleft obligation applies to the library itself and to derivative versions of that library, but not normally to independent programs that merely link to it.
  • Linking permissions: The license permits linking (dynamic or, under certain conditions, static) from non‑LGPL code without imposing the LGPL on the linking code, provided the recipient can replace or relink the library with a modified version.
  • Modifications: If you modify the library's source, those modifications must be distributed under the same license so that downstream users can benefit from and further modify them.

History and development

Originally called the "Library GPL," the name was changed to "Lesser" to emphasize that its copyleft is less restrictive than the GNU General Public License (GPL). The FSF produced several versions; the most commonly referenced are LGPL version 2.1 and LGPL version 3. Technically, LGPLv3 is implemented by applying additional permissions to GPLv3, which achieves the same practical effect while maintaining compatibility with the FSF's drafting practices.

Use, compliance and examples

Projects select the LGPL when they want their libraries to remain free but also to be usable by proprietary and closed‑source applications. Compliance typically requires distributing the library's source or offering it on request, and providing a means for recipients to relink the application against a modified library (for example, by using dynamic linking or supplying object files). Many system and utility libraries have used the LGPL or similar weak‑copyleft terms to encourage wide adoption while protecting library improvements.

Compatibility and notable distinctions

The LGPL differs from permissive licenses (such as MIT or BSD) in that it enforces copyleft on modifications to the library, and it differs from the GPL in that it does not automatically impose copyleft on all code that links to the library. These differences affect interoperability, commercial use, and contributor expectations. For authoritative explanations and the full licence texts, consult the FSF resources and license comparisons: LGPL information, Free Software Foundation licensing, license comparisons.

Practical advice

When choosing a license, consider your goals for reuse and contribution. If you want maximal adoption with minimal restrictions, a permissive license may be preferable; if you want to ensure improvements to your library remain free but you also want it to be linkable by proprietary software, the LGPL is often appropriate. As with any legal instrument, projects frequently consult legal guidance or the FSF documentation to confirm how the license applies to specific distribution scenarios.