Copyleft

Nevertheless, you should bear in mind that only “copyright” has any legal standing ... and the same thing goes for its symbol.

If you do decide to use this (legally unrecognized) symbol, I suggest that you also accompany it with legally recognized words such as (under U.S. law ...) Copr.   The law expressly authorizes the use of these abbreviations, in addition to the circle-right-facing-C symbol, and doesn’t care which one(s) you use as long as you do use one.   This fulfills the “notice” requirement.

In the eyes of the law, “Copyleft” is undefined.   Legally, it is a form of Copyright Licensing that you, as a copyright owner, are freely entitled to grant ... calling it under “a cute name” for your own amusement.   But, in order to be able to grant any rights at all, you should follow all the legal copyright requirements of your applicable system of law.   Formally register your work.   (In the USA, that costs $35.00 ... for a “collection” of as many works as you may wish to register at the same time for one fee.   See http://www.copyright.gov.)

(The notion of a “collection” is simply a cost-and-paperwork saver.   Protection applies equally and severally to each work, no matter how it was registered.)

Next, instead of relying upon a symbol that the reader might or might not understand (or even notice ...), state the terms (or, refer to an existing license agreement document) under which you are granting rights under your copyright.   Make it absolutely clear to them, without belaboring the point obnoxiously, what they may and may not do.   Don’t assume that they will even notice that the “C” is reversed, let alone know what you intend it to mean.   Kindly show them the courtesy of removing all doubt.

“Copyright” is simply a body of law that defines and secures your rights.   It says that you are therefore allowed to “grant” those rights, but it never says exactly how you must do it.   Licensure can be restrictive or permissive, or even both at the same time.   As the legally-recognized owner, it is up to you, and the full power of law-enforcement will, if need be, come to your aid.

By formally securing and registering your rights, you also reduce the potential legal liability of any subsequent user of your work.   (Who can and should register his “derivative work,” referencing your copyright-certificate number in his application to document the chain of ownership claims and provenance.)   No third-party person will be able to come out of the woodwork just to cause trouble (as they sometimes do ...), because the “chain of title” is clear and formal, as any judge can plainly see.   Given the draconian power of copyright law, and therefore the expense of dealing with it, this “minding your P’s and Q’s” is important for everyone ... and well worth a measly $35.00.