Once a creative idea has been given a viewable, tangible expression, it is immediately copyrighted. It is not necessary to place a copyright notice (c with a circle around it) on a work for it to be copyrighted. It is important, however, to note that:

Copyright rules vary if the work was created before April 1, 1989 (when the copyright law was changed by Congress into its current form). Even though it is not required, placing a copyright notice on a work does serve as a warning. It is possible to register the work with the U.S. Copyright Office.

Authors registering their works are entitled to increased damage rewards if copyright is infringed.