Currently, there are 2 National Championships each year, but the Spring Championships have traditionally been of a significantly lower level of competition than the Summer Championships. This is because the Spring Championships are so close to NCAA Championships and the fact that Spring Championships are rarely used as a selection meet for national teams. In many other sports, the National Championship of the sport is known as the "US Open" and while swimming did have a very high national level meet by that name each year, it was just a high level meet and not a national championship meet. This specific meet was ended in 2006 and was replaced with a reformulated Spring/Winter National Championship. Since there is no "US Open" meet of the old format, the National Championships (specifically Summer '08) have begun to be called the "US Open" to bring it in line with the nomenclature of other sports. The Olympic Trials are held once every 4 years. Since this meet offers such a coveted prize (a spot on the US Olympic Team) it never fails to attract the absolute fastest in the sport of swimming in the United States. Because of this, the entry time standards are even faster than the National Championships. However, even though this is a faster meet and would actually offer a truer indication of who is the fastest swimmer in the United States, the winner of each event in this meet is not officially considered a National Champion and this meet is NOT held in place of the National Championships every 4 years (although the Nationals are generally not held when the Olympic Trials occur, or other selection trials). However, for 2008, the winners of the Olympic Trials were indeed officially be a National Champion with the trials meet taking the place of the National Championship meet for 2008. It is unclear if this will continue for future trials. Unlike all other US Swimming meets, United States citizenship is required to compete at this meet since only United States citizens are allowed to represent the United States at the Olympics. The Olympic Trials are also under unique requirements made by the USOC. Trials meets are also held for the World Championships, Pan American Games and World University Games, typically at a national championship meet.