Doubles contain individual bets between 2 of the selections (legs) for all possible combinations.
At least 2 selections must win to earn a return.
Example
If there are 3 legs selected (A, B & C):
Bet 1: A x B
Bet 2: A x C
Bet 3: B x C
Example
If there are 4 legs selected (A, B, C & D):
Bet 1: A x B
Bet 2: A x C
Bet 3: A x D
Bet 4: B x C
Bet 5: B x D
Bet 6: C x D