In the United States, copyright has been made automatic (in the style of the Berne Convention) since March 1, 1989, which has had the effect of making it appear to be more like a property right. Thus, as with some forms of personal property, a copyright need not be granted or obtained through official registration with any government office. Once an idea has been reduced to tangible form, for example by securing it in a fixed medium (such as a drawing, sheet music, photograph, a videotape or a letter), the copyright holder is entitled to enforce his or her exclusive rights
Absence of a copyright notice does not mean that the work is not covered by copyright. The creator of an original work instantaneously possesses its copyright when that work is created through "mental labor" and "fixed" in tangible form. Thus, a natural copyright exists from the time a work is invented or created, regardless of whether it has been registered with a particular Copyright Office.
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Note that there are many works that are not part of the public domain, but for which the owner of some proprietary rights has chosen not to enforce those rights, or to grant some subset of those rights to the public