The GPL, first released in 1989, has become one of the best-known licenses because of its association with the Linux operating system. A distinguishing feature is that it requires derivative works to be distributed under the GPL, thus ensuring that once software is released under the GPL, it will remain open source permanently. In addition, works that contain GPL software must be themselves released under the license; if Microsoft wished to include a GPL-licensed utility in Windows, the entire OS would need to be made open source. "If you want to avoid having your software picked up and used in commercial products, then pick the GPL," said Bill Claybrook, president of research firm New River Linux & Grid Computing. These provisions created problems for software libraries, leading to the creation of the LGPLoriginally known as the Library GPL, its now generally referred to as the Lesser GPL. Under this license, the libraries themselves and any derived works must be distributed under GPL-type provisions, but software that merely uses the libraries can use another license.On the other end of the scale is the BSD (Berkeley Software Distribution) license, developed in 1977: It suggests, but does not require, that modifications to source code be returned to the developer community, and allows derived products to use other licenses, including proprietary licenses. BSD-licensed code can also be contained in, or can contain, code that uses other licenses. The flexibility of this license has allowed companies to create proprietary products based on BSD codeMac OS X, which is based on BSD Unix, is an example. The license is useful for companies wishing to encourage the broad adoption of their software in both the open-source and proprietary worlds. The MIT license is similar to the BSD license in its effects.Other licensesand there are manyfall between these two poles. The MPL, developed in 1998 when Netscape made its browser open-source, contains more requirements for derivative works than the BSD license, but fewer than the GPL or LGPL. AOL mixes MPL-covered components with proprietary software in its Netscape browser. Companies open-sourcing existing applications often create customized licenses, as Computer Associates has done with its Ingres database and the CA Trusted Open Source License. This allows third parties to incorporate Ingres into existing products, as long as the Ingres source is included with the product. Another popular scheme, adopted by MySQL, Trolltech, Sleepycat Software and others, is to release software under two licensesusually the GPL and a commercial license, according to Claybrook. Next page: Developer interest.