A double involves two selections. If the first selection wins, what would’ve been the return in a single bet instead all goes onto the second selection as the stake. To gain a return this second selection also has to win. If the first selection in the double bet loses then the double also loses. Where the odds of one or both selections are large enough it may be worth doing an each way double. In that scenario half of the stake would be on the win bet, and half on the place component, so if both parts of the bet placed, there would at least be some return.