The National Football League has historically been protective over unauthorized uses of its intellectual property, such as the game telecasts themselves, and most notably, its trademark for the Super Bowl—the league's championship game. To protect these properties and its official sponsors, the league has historically sent reminders and cease and desist notices to advertisers and businesses—including establishments that may be showing the game—that use references to "Super Bowl," "Super Sunday," or team names in promotional activities related to the Super Bowl. Although using part of a trademark for descriptive purposes, without implying official association, can be considered a nominative use under United States trademark law, non-sponsors typically use euphemisms such as "the Big Game" to refer to the Super Bowl in advertising to protect themselves from liability. In 2006, the NFL submitted an application to register "The Big Game" as a trademark as well, but withdrew following opposition by students of Stanford University and UC Berkeley, who play in a long-running college football rivalry game also known as "the Big Game"—an event which pre-dates the Super Bowl by several decades. The NFL has also partaken in clean zone policies around Super Bowl sites to protect against saturation ambush marketing.