4Clause

4Clause, commonly referred to as the original or four-clause BSD license, is a permissive software license historically used with the Berkeley Software Distribution (BSD) line from the University of California, Berkeley. It is part of the BSD family of licenses and is known for its four explicit clauses.

The license specifies four conditions. First, Redistributions of source code must retain the copyright notice, the list of conditions, and the disclaimer. Second, Redistributions in binary form must reproduce the same notices in the documentation and/or other materials provided with the distribution. Third, all advertising materials mentioning features or use of this software must display an acknowledgement stating that the product includes software developed by the University of California, Berkeley and its contributors. Fourth, the name of the University or its contributors may not be used to endorse or promote products derived from the software without prior written permission.

Originating in the late 1980s, the four-clause BSD license accompanied early BSD releases and was widely used in BSD-derived software. While its permissive terms allowed free use and redistribution, the advertising clause (the third clause) created practical and legal difficulties for larger projects and broader marketing efforts, leading to compatibility and maintenance concerns.

In the 1990s the BSD community moved toward the three-clause and later two-clause BSD licenses, which retained permissiveness while removing or relaxing the advertising and endorsement restrictions. As a result, the four-clause license is now largely considered historical and is rarely used for new software. It remains of interest primarily for understanding the evolution of BSD licensing and the history of open source software.