Contributions to Wikipedia are covered by the GNU Free Documentation License (GFDL), one of the many copyleft licenses that permit the redistribution, creation of derivative works, and commercial use of content. This license allows Wikipedia's editors to retain copyright of their work, but allows others to build upon that work and republish it, as long as the original author is credited and the derivative work is likewise covered by the GFDL. This license ensures that Wikipedia can be edited freely; each author's contributions are recorded for posterity in a database. Larry Sanger has said that having the GFDL as a ""guarantee of freedom is a strong motivation to work on a free encyclopedia"".