The United States Postal Service (USPS) has announced that Sylvester and Tweety were the most popular stamp of 1998. Over 37.9 million commemorative stamps of the bird and cat duo were saved by collectors. The USPS measures a stamp's popularity by the number saved - not used - through quarterly surveys sent to American households. The Postal Service issues 40-45 billion stamps in an average year, the majority of which are intended for day-to-day usage. Other top stamps included the Celebrate the Century sets from the 1900s, 1920s and 1930s, as well as the Classic Movie Monsters stamp set. The Sylvester and Tweety pairing is the second time a Warner Bros. cartoon character has been featured on a stamp, the first being the Bugs Bunny stamp released in 1997.