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While the above criteria are met by some public domain software (i.e., software that disclaims all copyright), much of what is called free software is copyrighted and licensed, with the rights of coping, modifying and redistributing the software explictly given to the user. One free software license in particular, the GNU General Public License, or GPL, requires that all modified versions of the software must be distributed with the source code, for the purpose of ensuring that all modified versions also meet the above criteria. See free software license, open source license.
See also: Open source, Free Software Foundation