Brilliant Twenty-One

How to play

1. Choose a stake of 10, 25, 50, 100 Carats and press Deal.
2. You get two cards face up. The dealer gets one face up and one face down.
3. Get closer to 21 than the dealer without going over. Number cards count as their number, court cards as 10, and an Ace as 1 or 11.
4. Hit to take a card. Stand to stop. Double doubles your stake for exactly one more card. Split turns a pair into two hands, each with its own stake.
5. When you stand, the dealer turns over the hidden card and draws until reaching 17 or more.

The rules in full

• 6 decks of 52 cards, shuffled together. The shoe is reshuffled when a quarter of it is left.
• The dealer stands on every 17, including a soft 17 (an Ace counted as 11 plus six).
• A Brilliant (an Ace and a ten-value card as your first two cards) pays 3 to 2.
• If the dealer shows an Ace or a ten-value card, the dealer checks for a Brilliant straight away. If there is one, the hand ends and you lose only your first stake, unless you also have a Brilliant, which is a push.
• You can double on any first two cards, including after a split.
• You can split any two cards of the same value once, into two hands. Split Aces get one card each. A 21 made after a split counts as 21, not a Brilliant.
• There's no insurance and no surrender.
• If you leave the page in the middle of a hand, the hand is cancelled and your stake is returned.
• Cards come from crypto.getRandomValues via a Fisher–Yates shuffle of the whole shoe.

Pays

Basic strategy

Basic strategy is the move that returns the most, on average, for your two cards against the dealer's face-up card. It was first worked out in 1956 and doesn't need any card counting. Turn on Show the basic-strategy move in the game and it will suggest a move for every hand.

H Hit S Stand D Double, or hit if you can't Ds Double, or stand if you can't P Split

How we measured the RTP

The slot and the wheel have no choices, so their return can be counted exactly. Twenty-one depends on how you play, so we measure it instead.

We dealt 20 million hands through the same rules engine the game uses, playing basic strategy every time. Stakes added by doubling and splitting count as stakes. Out of every 100 Carats staked, 99.57 came back.

The script that runs the test is in the site's source as tools/simulate-21.mjs.

A short history of twenty-one

Games of "twenty-one" go back at least four centuries. Cervantes mentions veintiuna in a story written in the early 1600s, and vingt-et-un was popular in 18th-century France. British players knew it as pontoon.

The name blackjack came later, in the United States. In 1956 four US Army engineers, Baldwin, Cantey, Maisel and McDermott, published the first basic strategy. Edward Thorp's Beat the Dealer took it to a wide audience in 1962.

We call a natural 21 a Brilliant after the round brilliant cut. Its proportions were set out by Marcel Tolkowsky in 1919 to return as much light as possible. Our court cards carry letters, not portraits.