All copyleft software is free software, but not all free software is copyleft, since some free software may be distributed under permissive licenses that do not require derivative works to maintain the same freedoms or licensing terms.
The MIT License is a permissive free software license that isn't copyleft, as it allows proprietary modifications.
https://www.gnu.org/graphics/copyleft-sticker.en.html
>This is a scan of the “Copyleft (L)” sticker on the back of the envelope mailed from Don Hopkins to Richard Stallman on 1984. The envelope contained a 68000 manual that Don borrowed from Richard, that he was returning. The sticker inspired Richard to use the word “Copyleft” for licensing free software.
>attention:
>READ NOTE BEFORE OPENING!
>Copyleft (L)
>The material contained in this envelope is Copyleft (L) 1984 by an amoeba named “Tom”. Any violation of this stringent pact with person or persons who are to remain un-named will void the warrantee of every small appliance in your kitchen, and furthermore, you will grow a pimple underneath your fingernail. Breaking the seal shows that you agree to abide by Judith Martin's guidelines concerning the choosing of fresh flowers to be put on the dining room table. Failure to break the seal on a weekday is […]
