The Parlay and Its Arithmetic
A parlay joins several selections into one wager that requires all of them. The appeal is the payout, which grows quickly, and the difficulty is that the probability shrinks just as quickly.
Take three selections each priced at minus one ten, each implying a shade over fifty-two per cent. Multiplied together they imply a little under fourteen and a half per cent. A fair price for that combination would be near six to one. The common fixed payout for a three-team parlay is six to one on the nose, which sits below fair by enough to make the combined margin considerably larger than the margin on any one of the three prices.
That is the general shape. Combining wagers does not combine the margins gently; it compounds them. A book that takes four and a half per cent on a single price takes a good deal more than three times that on a three-part ticket.
None of which makes the parlay an unusual product. It is simply priced as an entertainment with a long shape, and its arithmetic is worth reading before rather than after.