How a parlay multiplies a price
A parlay combines several selections into one ticket. The prices multiply, so two 2.00 selections become 4.00, and three become 8.00. That multiplication is the appeal, and it is also the whole story: every leg must win, so the probability of the ticket is the product of the probabilities of the legs.
Multiplying prices looks like multiplying value. It is not. It multiplies variance, and it multiplies the operator's margin once for every leg you add.
The margin stacks
Every selection in a parlay carries its own margin. Combine four legs and the compounded margin is materially larger than on a single bet. This is why a parlay often looks generous and prices out slightly worse than the equivalent straight bets — and why books promote them so heavily.
Ties, voids and reduced legs
A parlay leg that ties or voids does not usually kill the ticket. Instead, the leg is removed and the remaining legs are recalculated at their original prices. That sounds generous, but it means a "winning" parlay can pay considerably less than the advertised figure. Read this rule before you build, not after you win.
Correlation: when legs are not independent
Two selections from the same game are often correlated — a team winning and the total going over, for example. Correlated legs move together, which means the true combined probability is higher than the multiplication implies. Some books restrict correlated combinations in same-game parlays; others price them with an adjusted multiplier. Either way, the headline price overstates the difficulty of the ticket.
Where parlays fit in a staking plan
Because variance is multiplied, a parlay should be treated as a higher-risk, lower-stake instrument. If a straight bet is one unit, a four-leg parlay at the same unit size carries a very different risk profile. Many disciplined bettors cap parlay stakes below their standard unit precisely for this reason.
Round robins and other combinations
Some books offer round robins, which break a group of selections into every smaller combination. This produces many tickets from one idea, which raises total stake and total margin while lowering the chance of losing everything. It is a legitimate structure, but it is not a way to reduce risk — it is a way to redistribute it.