The GIF format uses the LZW compression algorithm, which was patented by Unisys, and publishers of applications that read and write GIF87a and GIF89a images were required to obtain a license and pay royalties to the company. For example, publishers of Web browsers and image editors were licensees. The Unisys patents expired in June 2003 for the U.S. and in July 2004 for Canada and Europe; however, Unisys and other companies have patents on different variants of LZW used in other GIF formats. See
LZW and
GIF.