The Triple Crown of U.S. thoroughbred racing is a prize given to a horse that wins the Kentucky Derby, the Preakness Stakes, and the Belmont Stakes in the same year. Horses that win the Triple Crown are usually ridden by the same jockey, and both horse and jockey are awarded the Triple Crown Trophy.

The three races are all run within six weeks of one another, in May and early June. The Kentucky Derby takes places in Louisville, Ky., on the first Saturday in May, the Preakness follows two weeks later in Baltimore, Md., and the Belmont rounds out the schedule three weeks after that, in Belmont, N.Y.

This was not always the case. The Belmont Stakes was the first of the three races to be run, in 1867. The Preakness came along in 1873, followed by the Kentucky Derby two years later. In 1917 and 1922, the Kentucky Derby and the Preakness were run on the same day. The current order was set in 1931.

Winning the Triple Crown is a difficult achievement. As of 2015, only 12 horses have done it.

The 12 horses that have won the Triple Crown are these:

• Sir Barton (1919)
• Gallant Fox (1930)
• Omaha (1935)
• War Admiral (1937)
• Whirlaway (1941)
• Count Fleet (1943)
• Assault (1946)
• Citation (1948)
• Secretariat (1973)
• Seattle Slew (1977)
• Affirmed (1978)
• American Pharaoh (2015).

One reason that winning the Triple Crown is so difficult is that the three races are of different lengths. The Kentucky Derby race distance is 1.25 miles. The distance for the Preakness Stakes is 1.19 miles. The longest of the three, the Belmont, is 1.5 miles.

The history of these three races is full of horses that have won one race or even two in the same year but not all three. In recent years, trainers have opted not to enter horses in all three races. American Pharaoh, the 2015 Belmont winner, was the first winner of that race in nearly a decade to have run the first two races as well.

Other notes:

• Jockey Eddie Arcaro, the only rider to win the Triple Crown twice, rode both Whirlaway (1941) and Citation (1948). Both horses were owned by Calumet Farms.
• Omaha, the 1935 Triple Crown winner, was the son of 1930 Triple Crown winner Gallant Fox. Both horses had the same owner, Belair Stud, and the same trainer, James “Sunny Jim” Fitzsimmons.
• The race record for each of the three races is still by held 1973 winner Secretariat. The 2 minutes, 24 seconds that the horse turned in for the 1937 Belmont Stakes is still a world record for that distance.
• The youngest jockey to win the Triple Crown was Steve Cauthen, who was 18 when he rode Affirmed to three victories in 1978.
• Victor Espinoza, the jockey who rode 2015 Triple Crown winner American Pharaoh, was the oldest jockey to win the prize, at 43.
• Trainer D. Wayne Lukas had one horse, Thunder Gulch, win the Kentucky Derby and the Belmont in 1995 and another horse, Timber Country, win the Preakness in the same year.
• More than 4,100 horses have competed in the three Triple Crown races. Nearly 300 horses have won one of the races. Only 52 have won two (but not all three). Of those two-win combiantions, 23 were the Kentucky Derby and the Preakness, 18 were the Preakness and the Belmont, and 11 were the Kentucky Derby and the Belmont.
• Alydar finished second to Affirmed in all three races in 1978.
• The Kentucky Derby has run every year since its inception, in 1875. The Preakness did not run in 1891–1893, and the Belmont did not run in 1911–1912.