derivative code same license Copyleft Modified from http://www.gnu.org/copyleft/copyleft.html, accessed 12 June 2013. Copyleft is a derivative code same license clause which says that anyone who redistributes the software, with or without changes, must pass along the freedom to further copy and change it. In other words, it requires that all derivative code uses the same license, but further limits the type of license to one which gives everyone the rights to use, modify, and redistribute the program's code, or any program derived from it, but only if the distribution terms are unchanged. To copyleft a program, you first state that it is copyrighted; then the distribution terms described above are added. This makes copyleft a legal instrument ensuring that the code and the freedoms become legally inseparable. Allyson Lister