Z:gnu-www-ja-google-engineering-talk--3c5340-Copyleft makes the four freedo/en

Copyleft makes the four freedoms into inalienable rights for all users, so that wherever the code goes, the freedom goes with it. The specific license that we use to implement the general concept of copyleft is called the GNU General Public License, or GNU GPL for short. This license is used for around two thirds or three quarters of all free software packages. But that still leaves a substantial number that have other licenses. Some of those licenses are copyleft licenses, some are not. So we have copylefted free software and we have non-copylefted free software.