BSD 4-Clause License

The BSD 4-clause license is the original BSD license, used for the Berkeley Software Distribution, a Unix-like operating system. It contained a clause not found in later licenses, known as the advertising clause, which required authors of all works deriving from a BSD-licensed work to include an acknowledgment of the original source in all advertising material. The clause became controversial: Richard Stallman stated that he counted 75 such acknowledgments in a 1997 version of NetBSD, and the clause is incompatible with the GPL, so the Free Software Foundation recommends that developers not use the license, though it states there is no reason not to use software already using it.