parlay

A parlay, accumulator (or acca), combo bet, or multi is a single bet that links together two or more individual wagers, usually seen in sports betting. Winning the parlay is dependent on all of those wagers winning together. If any of these bets in the parlay lose, the entire parlay loses. If any of the plays in the parlay ties, or "pushes", the parlay reverts to a lower number of wagers with the payout odds reducing accordingly. Parlay bets are high-risk, high-reward; linking the possibilities drastically reduces the chance of the bet paying off overall. The benefit of the parlay is that there are much higher pay-offs, although as usual, casinos and bookkeepers offering parlays often exploit the poor calculation of gamblers by not increasing the pay-out as much as the odds truly demand, with the effect of the house edge increasing in parlays.
Although a variety of bets can be used to build a parlay bet, correlated parlays are usually not allowed by traditional bookmakers. Correlated parlays are two or more bets from the same game that rely on a closely related outcome: for example, betting that a football (soccer) team might both score more than three goals in a match, and also win the match. These are not independent events, as a team that scores more than three goals is also very likely to win the match. A naive application of odds that treated these events as uncorrelated would not accurately reflect the probability of the linked bet. However, with the rise of sports betting through mobile gambling apps in the late 2010s–2020s, this traditional hesitance has weakened. These newer apps often allow "microbets" on propositions such as if the current possession will end in a score, and further allow these props to be bundled into parlays. The system then attempts to compensate for the degree of correlation. While these systems usually prefer to lean heavily in the house's favor, the increased volatility has resulted in some notable cases where bettors have found favorable odds. This does not always a windfall for the bettors, as sportsbooks have sometimes simply refused to pay out when such parlays come through.
While parlays were traditionally an uncommon side-bet, they have become more common in sports betting apps of the 2020s. Due to the larger house edge on parlays, they now account for over 60% of the revenue of some sports betting companies, a substantially larger proportion than the number of bets by volume.

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