Parlays and correlation

A parlay multiplies bets together and every leg must win. The payout grows fast, which is the attraction. So does the book's cut, which is not advertised — each leg carries its own vig, and combining them multiplies the margins as well as the odds. That is why books promote parlays relentlessly and almost never promote sides.

The rule that actually matters

A parlay of +EV legs is still +EV. If every leg beats its fair price, parlaying them keeps the value — you are just accepting far more variance for it. Anyone telling you never to parlay is telling you never to bet a legitimate edge because they dislike the shape. Bet straight if you want smoother results; the value does not disappear either way.

Same-game parlays and correlation

Legs in the same game are not independent. A star scoring 30+ makes his team winning more likely, so a book prices that pair below what the two prices multiply to — perhaps +280 where separate games would give +430. That is the book charging you for the correlation, and it is usually fair.

The opportunity is where they get it wrong: a book pricing correlated legs as though they were unrelated is handing you value. The trap is the reverse — legs that fight each other, like a player's under with his team's over, which quietly destroys a ticket that looked fine.