Shared universes often come about when a fictional universe achieves great commercial success and attracts other media. For example, a successful movie may catch the attention of various book authors, who wish to write stories based on that movie. Under U.S. law, the copyright-holder retains control of all other derivative works, including those written by other authors, but they might not feel comfortable in those other mediums or may feel that other individuals will do a better job; therefore, they may open up the copyright on a shared-universe basis. The degree to which the copyright-holder or franchise retains control is often one of the points in the license agreement.