Z:gnu-www-ja-categories--5e2ba4-Copyleft is a general concept;/en

Copyleft is a general concept; to copyleft an actual program, you need to use a specific set of distribution terms. There are many possible ways to write copyleft distribution terms, so in principle there can be many copyleft free software licenses. However, in actual practice nearly all copylefted software uses the GNU General Public License. Two different copyleft licenses are usually &ldquo;incompatible&rdquo;, which means it is illegal to merge the code using one license with the code using the other license; therefore, it is good for the community if people use a single copyleft license.