CCO and Public Domain
When a work is in the public domain , it is free for use by anyone for any purpose without restriction under copyright law. Public domain is the purest form of open/free, since no one owns or controls the material in any way
Works that are in the public domain in one legal jurisdiction are not necessarily in the public domain worldwide. Copyright laws differ from jurisdiction to jurisdiction, both in duration of protection and what constitutes copyrightable subject matter. For example a US Government work clearly in the public domain in the United States may or may not be free of copyright restrictions and in the public domain in other jurisdiction. At present, one of the only ways to be certain that a particular work is in the public domain worldwide is to see if the copyright holder has dedicated all rights to the work to the public domain by using CC0.
Creative Commons licenses do not affect the status of a work that is in the public domain under applicable law, because our licenses only apply to works that are protected by copyright. For more information, see our Licensing Guide to what you should know before you license a work using CC licenses.
CC0 is the "no copyright reserved" option in the Creative Commons toolkit - it effectively means relinquishing all copyright and similar rights that you hold in a work and dedicating those rights to the public domain.
CC0 is a single purpose tool, designed to take on the dedication function of the former, deprecated Public Domain Dedication and Certification.
How effectively CC0 works will depend on the legal regime in which the work is used, but the tool is intended to effectively release rights even in jurisdictions where it is difficult to do so.
Note that CC0 is a three-tier instrument. We recognize that a waiver may not be effective in some jurisdictions. CC0's enforceability is not solely dependent on the waiver. The fall back public license -- the second tier -- is similar to our Attribution-only license but without the attribution requirement. The third tier is a non-assertion by the copyright holder that even if the waiver and license do not operate as intended, the copyright holder will not take any actions that prevent a user of the work from exercising rights consistent with the intention of the copyright holder as expressed in CC0.