Filling a slip for "What are decimal odds?" starts with one ticket. A parlay, called an accumulator in many places, joins several picks into one bet. Every leg has to win for the ticket to pay, and the odds of the legs multiply, so a small stake can return a large sum. What makes the format attractive is the same thing that makes it hard: each added leg lowers the chance that the whole bet comes in.
A parlay works by multiplication. Each leg carries its own odds, and the payout on a parlay is the stake times the decimal odds of every leg multiplied together. Three legs at 2.0 each turn one unit into eight. The same arithmetic runs the other way for the chance of winning: three picks that each land half the time win together only one time in eight, and the price offered usually pays less than that fair eight.
Can parlays be profitable over a long run? Rarely. Each leg carries the book's margin, and multiplying legs multiplies the margin as well, so a four leg accumulator hands the house far more edge than four single bets would. Accumulator bets are worth it as entertainment priced in advance, a small stake on a long shot. Anyone who treats them as a way to earn is paying the widest margin on the menu.
Each leg multiplies the odds, but also the book's margin and the chance of failure, so three picks landing half the time win together only one time in eight.
A small stake treated as entertainment money priced in advance, since multiplied margins make accumulators a poor way to earn.
A round robin splits a list of picks into every smaller parlay, such as doubles or trebles, costing one stake per combination so a single loss does not sink everything.
Multiply the stake by the decimal odds of every leg; three legs at 2.0 each turn one unit into eight.
Most books drop that leg from the ticket, and the parlay pays at reduced odds on the legs that remain.
A bet that reverses the usual logic and pays out when at least one of the selected legs loses.