The GNU Lesser General Public License (LGPL) is another license created by the FSF for the purpose of permitting a certain class of programs, generally subroutine libraries, to be licensed under an FSF license but be permitted to link with non-GPL software programs. Subroutine libraries provide various functions to other programs, and because as part of their function they link with such programs, the resulting program plus library could be considered as a legal matter to be a derivative work. Accordingly, if the other program were licensed under a proprietary license and the library under the GPL and the program and library were distributed together under the proprietary license, the GPL would be violated, as the program plus library would be considered a derivative work that would be subject to limitations on copying, distribution, and modification that are inconsistent with the GPL.^[11]

The LGPL provides an alternative license that preserves many of the benefits of the GPL model for such libraries—in fact, the Lesser General Public License was in its first incarnation known as the Library General Public License. LGPL-licensed libraries can be linked with non-GPL licensed programs, including proprietary software. However, libraries need not be licensed under the LGPL, and as the following preamble to the license points out, the preferable way to license libraries, at least under some circumstances, is under the GPL.