American copyright law traces its lineage to the English Statute of Anne, passed in 1710. The Statute provided that authors of books had an exclusive right to their works for fourteen years, which could be renewed by the author for an additional fourteen years.
After the American Revolution, many states passed statutes modeled on the Statute of Anne. Because the laws were different from state to state, the Constitutional Convention decided that the new Congress should have the power to pass a national copyright statute, and added a clause in Article I to that effect. The statute was enacted as the Copyright Act of 1790, one of Congress's first laws. Like the Statute of Anne, it provided a 14-year initial term, renewable for an additional 14 years, and it only applied to books, maps, and (maritime) charts. Over the next century, the 1790 Act was expanded to cover other types of works.
There was a full revision of the copyright laws in 1870, and after that the next major revision of the Act was passed as the Copyright Act of 1909. The 1909 Act doubled the term of copyrights to 28 years, renewable for an additional 28, and expanded protection to cover all written works in the US as well as many foreign works. Both the 1790 Act and the 1909 Act required a number of formalities before copyright would attach: these formalities are described later.
The Copyright Act of 1976 formed the basic copyright law regime which the US follows today. It provided a single copyright term for works created after 1977: the life of the author, plus fifty years (seventy-five years from fixation in the case of corporate authors). It eliminated many of the required formalities and applied copyright at the moment the work was "fixed" in a tangible medium of expression.
Since then, the main revisions to the 1976 Act have been the Audio Home Recording Act of 1992, adding new laws pertaining to digital audio recordings; the Sonny Bono Copyright Term Extension Act of 1998, extending the term of copyright for an additional twenty years (see Duration); and the Digital Millennium Copyright Act of 1998, imposing new rules on high-tech works.