LGPLv21

LGPLv21 refers to the GNU Lesser General Public License version 2.1, a free software license published by the Free Software Foundation in 1999 as an update to LGPL v2. It is a weak copyleft license designed for software libraries, intended to allow libraries to be used by non‑copyleft software while preserving freedom for users to study and modify the library itself.

As a library license, LGPLv2.1 permits linking with non‑LGPL software. This means that proprietary or differently licensed applications can use an LGPL‑covered library without having to release their own source code under the LGPL. However, if you modify the LGPL‑covered library, you must distribute the modified library under LGPLv2.1 (or a later version) and provide the corresponding source code for those modifications.

Key obligations include preserving the library’s notices and warranty disclaimer, and providing access to the library’s source code. When distributing the library, you must provide the corresponding source or offer to provide it, and you must supply installation information that would help a user relink the application with a modified version of the library if desired. If the library is distributed in object code, you must still ensure that users can replace the library with a modified version via relinking.

Relation to other licenses: LGPLv2.1 is designed to be compatible with non‑LGPL software and with GPL licenses, as it allows the library to be used in larger works without imposing GPL on the whole program. It was later superseded in practice by LGPLv3, which added clarifications and new terms, but LGPLv2.1 remains in use for many libraries today.