An "Accumulator" or "Acca" is a bet that combines four or more selections into a single wager that gains a return only when all parts win. The advantage of an accumulator is that winnings are much higher at the expense of increased risk, only a single selection need lose for the entire bet to lose.
n the case of a non-runner, returns are calculated as though the accumulator hadn't contained the selection i.e. a six-fold would become a five-fold. Dead heats for a winning position are settled at reduced odds using standard rules.
Selections from the same event cannot be combined into a single accumulator, this is to prevent anyone trying to place several bets on the same runner. More formally the selections must be mutually independent.