21 was based on Bringing Down the House: The Inside Story of Six MIT Students Who Took Vegas for Millions (2002) by American author Ben Mezrich. How accurate is the movie? The basic premise, that a group of MIT students and grads ran a card counting operation and would play in Vegas on the weekends, is true.
Blackjack, a fast-paced card game, historically paid out a ratio of 3:2 when a player hit 21 on the first two cards. That means a gambler wins $15 for every $10 bet. Now, many blackjack tables on the Strip pay out at 6:5, which means that same $10 yields only $12.