Under the Berne Convention, member states are free to prescribe in their national legislation that copyright is automatically granted to a work only when it is "fixed", that is, written or recorded on some physical medium. In the U.S., copyright law only protects works that are fixed in a tangible medium of expression. Accordingly, some argue, the processes or movements required for a trick are not copyrightable within the U.S, but recordings, written descriptions, or photographs of such a performance may be copyrightable themselves.