An accumulator combines several selections into one bet. The odds of each selection are multiplied together, so the possible payout grows quickly. The same multiplication applies to the chance of losing: one wrong selection ends the whole bet. Each added selection also adds the bookmaker’s margin again, so longer accumulators tend to cost more in the long run, even though the top prize looks bigger.
Example
Three selections at decimal odds of 2.00 each give combined odds of 8.00 (2.00 x 2.00 x 2.00). A 1 unit stake returns 8 units, a profit of 7. If each selection has a true 50% chance, all three win only 12.5% of the time.