Payout, combined odds, and the number the ads never show you — the implied chance that every leg actually lands.
Every leg of a parlay carries the book's cut, and the cuts multiply. Two standard -110 legs don't cost you twice the usual margin — they compound, and by six or seven legs the total cost is several times what a single bet carries. That's why the payout looks generous and the "chance all legs land" number looks the way it does.
A useful habit: before placing any parlay, look at that implied chance and ask whether you'd take that probability as a straight yes/no bet at the same price. If the answer is no, the extra legs didn't make it smarter — just longer.
The same math runs live on every game on today's board, with the book's cut stripped out so you can see what the market really thinks.