To copyleft a program, first we copyright it; then we add distribution terms, which are a legal instrument that 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. Thus, the code and the freedoms become legally inseparable.

Proprietary software developers use copyright to take away the users' freedom; we use copyright to guarantee their freedom. That's why we reverse the name, changing 'copyright' into 'copyleft.'

GNU software is freely distributed, but in a different manner from public domain and ''freeware'' software among personal computer users. While public domain software can be freely copied, freeware authors ask users to contribute a fee if they find a program useful. In contrast, GNU programs are not placed in the public domain. Instead they are distributed with a public license that Mr. Stallman calls a ''copyleft.'' This license insures that the software will stay freely copyable and not be incorporated into a for-profit program.