Free software is software that can be used, studied, and modified without restriction, and which can be copied and redistributed in modified or unmodified form either without restriction, or with restrictions only to ensure that further recipients can also do these things. To make these acts possible, the human readable form of the program (called the source code) must be made available. The source code may be either accompanied by a software license saying that the copyright holder permits these acts (a free software licence), or be released into public domain, so that these rights automatically hold.