Since 1919, only 13 horses have won the title of Triple Crown winner by finishing first in the Kentucky Derby, Preakness Stakes and Belmont Stakes in the same year.<br><br> Sir Barton, foaled at Hamburg Place Farm in Lexington, Ky., was the first Triple Crown winner in 1919, before the phrase "Triple Crown" was even coined. He's pictured here after winning the 44th running of the Kentucky Derby at Churchill Downs in May 1919.

Gallant Fox, with jockey Earl Sande who came out of retirement to race him, wears the roses after winning the 1930 Kentucky Derby. He retired to Claiborne Farm in Paris, Ky., where he sired 1935's Triple Crown winner Omaha.

Omaha gallops in his last hard workout before running the 1935 Preakness Stakes at Pimlico Race Track in Baltimore, May 10, 1935.

War Admiral and jockey Charley Kurtsinger are pictured at Belmont Park in Elmont, N.Y., June 5, 1937. "The Admiral," was unruly at the Belmont Stakes, crashing through the gate several times and delaying the race by several minutes. Stumbling out of the gate, he suffered an injury to his right front heel. After running to victory and the Triple Crown, he recuperated for a few months before returning to race for two more years.

Whirlaway is draped with the floral tribute in the winner's circle at Belmont Park, N.Y., June 7, 1941, after winning the Belmont Stakes race of the Triple Crown with jockey Eddie Arcaro. Trainer Ben Jones holds the bridle.

Count Fleet is posed with jockey Johnny Longden in New York, May 22, 1943. Longden rode Count Fleet to become just the sixth horse to win the Triple Crown.

Wearing a garland of Roses, Assault stands in the Kentucky Derby winner's circle, May 4, 1946, in Louisville, Ky., with Mrs. Robert J. Kleberg, Jr., Jockey Warren Mehrtens, and owner Robert J. Kleberg, Jr. of King Ranch, Texas.

Citation, ridden by Eddie Arcado, races to victory in the Belmont Stakes at Belmont Park race track in Elmont, N.Y. on June 12, 1948. The winning run tied previous Triple Crown winner Count Fleet's time of 2:28:20. Citation was the eighth horse to win the Triple Crown and eventually became the first to win over a million dollars in his racing career.

Jockey Ron Turcotte walks Secretariat towards the winners circle after they captured the Triple Crown by winning the Belmont Stakes at Belmont Park in Elmont, N.Y., June 9, 1973. Nicknamed "Big Red," the ninth Triple Crown winner set speed records at all three races.<br><br>Some consider his win at the Belmont Stakes to be the "single greatest performance in the history of horse racing."

Jockey Jean Cruguet sits on Seattle Slew in the winners circle after winning the Belmont Stakes to become the in Elmont, N.Y., June 1, 1977. The tenth horse to become a Triple Crown champion, he was the first to have won the title with a perfect winning record. Before retirement, he defeated the following year's Triple Crown winner, Affirmed.

Jockey Steve Cauthen rides Affirmed, who took an early lead during the Belmont Stakes on June 10, 1978. A mile into the race, Affirmed's rival Alydar made a valiant attempt to take first, but Affirmed held on to his lead by a narrow margin to win the Triple Crown with one of the fastest times in Belmont history.

American Pharoah crosses the finish line to win the 147th running of the Belmont Stakes horse race at Belmont Park, June 6, 2015, in Elmont, N.Y. American Pharoah was the first horse to win the Triple Crown since Affirmed won it in 1978.<br><br> Later in 2015, American Pharoah also won the Breeders' Cup Classic, which was established in 1984, inaugurating a winning sweep referred to as a Grand Slam.

Justify, ridden by Mike Smith, wins the Belmont Stakes to become the 13th Triple Crown champion at Belmont Park in Elmont, N.Y., June 9, 2018.