(CNN) They form the bedrock of the British Flat racing season and are the benchmark on which careers and legends are based.
The Classics identity the stars of their generation and provide the yearly narrative around which racing revolves.
The 2,000 Guineas, the 1,000 Guineas, The Oaks, The Derby and the St. Leger are the races that really matter, like majors in golf or grand slams in tennis.
And they have serious history. The St. Leger began in 1776.
They became collectively known as the Classics in 1815, and ever since have crowned the best three-year-olds in training.
With the Guineas taking place at Newmarket this weekend, CNN Sport takes a closer look at the Classics.
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The 2019 Classic season kicks off Saturday with the 2,000 Guineas over the turf of the Rowley Mile racecourse in Newmarket, Suffolk, traditionally known as the home of English Flat racing.
The first recorded race in the town was a match for £100 between horses owned by Lord Salisbury and the Marquess of Buckingham in 1622.
The 2,000 Guineas is a straight mile test for thoroughbred three-year-old colts and fillies and was first run in 1809. It was instigated by Sir Charles Bunbury, who had earlier established The Derby at Epsom.
The name refers to the original prize fund -- a Guinea was equivalent to 21 shillings or £1.05. The purse is now £500,000 ($682,800) with £283,550 ($365,600) going to the winner.
The 2,000 Guineas Stakes, as it is properly known, represents the first leg of the English Triple Crown, alongside The Derby and the St. Leger. But the demands for both speed and stamina -- from the mile of the Guineas to the mile-and-a-half of The Derby and the mile-and-three-quarters of the St. Leger -- mean the feat is rarely attempted.
The last to horse to clinch the Triple Crown, and the only one since World War II, was Nijinsky in 1970.
Since then only Nashwan (1989), Sea the Stars (2009) and Camelot (2012) have won the Guineas and the Derby. The great Frankel, 2,000 Guineas winner in 2011, didn't even attempt it.
Irish trainer Aidan O'Brien took his record of wins to nine when his son Donnacha rode Saxon Warrior to victory last year.
The Guineas kicks off the 2019 British Champion Series, a season-long circuit encompassing some of the biggest prizes in UK Flat racing.
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The race is held over the same course at Newmarket a day after the 2,000 Guineas but this is for fillies only.
It was first staged in 1814 and represents the first leg of the fillies' Triple Crown -- with The Oaks and St. Leger -- but again this is rarely attempted.
The last horse to win the fillies Triple Crown was Oh So Sharp in 1985.
As the 2,000 Guineas can act as a trial for The Derby, so the 1,000 Guineas can be seen as a trial for The Oaks. The O'Brien-trained Minding was the last horse to win both in 2016, but the first since Kazzia in 2002.
Billesdon Brook, trained by Richard Hannon, became the biggest-priced winner in the history of the race when she triumphed as a 66-1 shot in 2018.
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The third Classic of the season kicks off The Derby meeting at Epsom, south of London, in early June.
The Oaks was the second Classic to be established -- in 1779 -- and was named after an estate near Epsom leased to the 12th Earl of Derby in the 18th century. Racing was first recorded at Epsom in 1661.
The race, for fillies only, is a step up in distance from the Guineas, run over a mile-and-a-half on the undulating Downs.
The course -- the same as for The Derby the following day -- is roughly U-shaped with a climb from the start, followed by a sweeping left-hand turn (Tattenham Corner) into a mostly downhill straight, finishing with a stiff climb in the last few hundred yards.
Enable, which went on to win back-to-back Prix de l'Arc de Triomphe titles in Paris, holds the race record of two minutes 34.13 seconds.
The purse is £500,000.
Derby Day is the pinnacle of the British Flat racing season and a highlight of the cultural and sporting calendar.
Since its inception in 1780 it has attracted both the upper echelons of English aristocracy -- the Queen herself has only missed two runnings since 1946 -- along with huge crowds made up of every level of society for a colorful day out on the Downs. In 2017, more than 150,000 attended the two-day Festival.
The Derby was named after Lord Derby, but the tale goes the naming rights came down to a coin toss between him and Sir Charles Bunbury during a party where the idea for the race was formed.
The Derby's long history features the 1913 edition, dubbed the "Suffragette Derby," when women's rights campaigner Emily Davidson threw herself under the thundering hooves of King George V's horse Anmer and died four days later.
The mile-and-a-half (1m 4f) test, for both colts and fillies, identifies the supreme all-round horses of their generation and has spawned superstars such as Sea Bird, Nijinksy, Mill Reef, Nashwan, Shergar (who won by a record 10 lengths) and Galileo.
Only six fillies have won The Derby in its history, most recently Fifinella in 1916.
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Legendary jockey Lester Piggott won a record nine Derbys between 1954 and 1983.
"Epsom is the complete test of a horse," said O'Brien, who has saddled six Derby winners including Galileo, now one of the world's most valuable horses at stud.
"They need balance, speed and stamina. It's uphill, downhill, sideways. It's a very intense atmosphere."
The Derby is Britain's richest race and is worth £1,500,000 ($1.93 million) with the winner receiving £850,650 ($1.1M).
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The final Classic of the season takes place at Doncaster in the north of England in September and is raced at the extended distance of a mile-and-three-quarters (1m 6f).
The left-handed race, for three-year-old colts and fillies, was devised by Anthony St. Leger, a local army officer and politician.
The first running of what was then a two-mile race was known as "A Sweepstake of 25 Guineas," and was held at Cantley Common in 1776.
The name was changed to reflect St. Leger's influence the following year, but in 1778 it moved to Town Moor, which has a racing history going back to the 16th century.
The race has variously been staged at Newmarket, Manchester, York, Thirsk and Ayr because of war, subsidence and redevelopment.
The St. Leger commands a purse of £743,000 with £421,355 going to the winner.