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A daily double is a pari-mutuel wager offered by horse and dog racing tracks (and at jai alai frontons.) Bettors wager on the winners of two races, pre-designated by the track for a particular race day. While the Daily Racing Form's glossary defines a Daily Double as two consecutive races,  this is a loose rule. Many tracks' Daily Doubles are not consecutive. Because of the increased difficulty of picking two straight winners, winning daily double bets pay off at higher odds. 
The daily double was the first so-called "exotic" wager offered by North American racetracks. Introduced in 1931 at Ottawa's Connaught Park Racetrack, the wager was typically offered only for the first two races of each day's program as an enticement for spectators to arrive early for the entire program. As with all other American racing wagers, the "double" is conducted in parimutuel fashion, but with the number of betting interests in the daily double pool equal to the product of the number of entries in each race. For example, if there are ten entries in the first race and eight in the second, there will be eighty betting interests, one for each combination of two potential winners. This results in higher payoffs than those found in straight betting for win, place, or show.
For many years the daily double was the only exotic wager offered. Later the exacta was also offered on select races during each program. The wagers were offered only a few times on each card, largely because of the limitations of electro-mechanical totalisator systems. When computer technology took over, more-exotic wagers were introduced, such as the trifecta, superfecta, and pick 6. The higher payouts for these wagers tended to diminish interest in the "old fashioned" daily double, but it is still offered at all tracks, sometimes more than once during a program. A "late double" is frequently offered on the day's final two races; some tracks offer a "rolling double" - a daily double starting on each race on the program except the last race.
The "Pick 3" and "Pick 4" wagers are derived from the daily double. These wagers require bettors to pick the winners of three or four consecutive races. These are also often offered on a rolling basis — a rolling pick 3 on races one through three, another on races two through four, and so on throughout the day.
Occasional doubles are offered on important races contested on separate days. The most prominent example is the "Oaks-Derby Double" offered by Churchill Downs, where bettors pick the winners of the Kentucky Oaks and the Kentucky Derby. The Oaks is run the day before the Derby; the latter is always run on the first Saturday of May.