A double is the simplest multi-selection bet — two picks combined into a single wager. Both selections must win for the bet to pay out. The odds of both selections are multiplied together to give the combined odds, and the stake is applied to that combined price.
The mechanism works like a parlay: the full return from the first selection (stake × odds 1) becomes the effective stake on the second selection. If Selection 1 wins at 2.0 with a £10 stake, the £20 return is entirely reinvested on Selection 2 at 3.0, giving a final return of £60.
Doubles vs singles is a classic question of risk preference. Placing two separate singles on the same selections costs the same total stake but gives you a return even if only one wins. The double requires both to win but returns more when they do. Over large volumes of bets, two singles generally produce more consistent returns because they allow partial recovery.
Doubles are popular in football, horse racing, and greyhound racing. In horse racing, a daily double (first and second race at a meeting) is a specific fixed market offered by many bookmakers and tracks.
Example
Saturday football: you back Manchester City at 1.50 and Real Madrid at 1.80. Combined double odds = 1.50 × 1.80 = 2.70. A £10 double returns £27 if both teams win. If City win but Madrid draw, the bet loses entirely — you receive nothing, whereas two £5 singles would have returned £7.50 from the City single.