Under copyleft, copyright infringement may be avoided if the would-be infringer perpetuates the same copyleft scheme. For this reason copyleft licenses are also known as viral or reciprocal licenses.

when Richard Stallman was working on a Lisp interpreter. Symbolics asked to use the Lisp interpreter, and Stallman agreed to supply them with a public domain version of his work. Symbolics extended and improved the Lisp interpreter, but when Stallman wanted access to the improvements that Symbolics had made to his interpreter, Symbolics refused. Stallman then, in 1984, proceeded to work towards eradicating this emerging behavior and culture of proprietary software, which he named software hoarding.[2]

he created his own copyright license, the Emacs General Public License > [3] > , the first copyleft license. > This later evolved into the GNU General Public License