[Diesen Artikel gibt es auch auf Deutsch.]
It’s probably safe to say that the copyleft principle has been essential for the success of free software. Copyleft means that all versions of a software or document will remain free, preventing companies from creating “value-added” versions of free programs and selling them as proprietary, non-free software. The GNU General Public License (GPL)—the first and most well-known incorporation of the copyleft principle—is used for about 50–70% of all free programs, making it more popular than all other free software licenses together.
At first sight, the situation in the newly emerging field of free and open hardware might seem similar—here, copyleft licenses such as the GPL and the Creative Commons Attribution-ShareAlike License (BY-SA) are very popular too (see below for a more detailed analysis). But actually, the situation is very different for hardware design, since copyleft relies on copyright, and hardware is (in most cases) not protected by copyright law.
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