Translations:Essays:Why are GPL-licensed components removed from the FreeBSD base system/6/en

Copyleft licenses

It was important for Stallman not only to reflect in the license the four basic freedoms mentioned earlier in the article but also to ensure that once it became publicly available, the code could not be taken away and appropriated. Thus, if someone takes software that is covered under the GPL as a basis and builds something new from it, or just fixes bugs, the derivative work must also be distributed under the GPL. So, once released under the GPL, a piece of code can no longer be closed. Stallman called this legal mechanism copyleft, i.e. copyright turned inside out. The first versions of the GPL came out in the late 80s and early 90s and became very popular.