Blackjack deviations: when to break basic strategy

Basic strategy is the best play when you know nothing about the cards left in the shoe. A counter does know something. Deviations, also called index plays, are the spots where the true count says a different play is now better.

How index numbers work

Each deviation has an index: the true count at which the correct play changes. At or above the index, you make the deviation. Below it, you stick with basic strategy.

Take 16 against a dealer 10. Basic strategy says hit. With Hi-Lo, the index is 0: at a true count of 0 or higher, stand instead. A positive count means more tens are left, so hitting a 16 is more likely to bust.

Some deviations work in the other direction. Basic strategy stands on 12 against a dealer 4, but when the true count drops below 0, the shoe is full of small cards and hitting becomes the better play.

The Illustrious 18

In Blackjack Attack, Don Schlesinger ranked the deviations that capture most of the value of playing to the count. He called the top group the Illustrious 18. These are the first ten:

Make the deviation at the index or higher. These are commonly published Hi-Lo indices for multi-deck games where the dealer stands on soft 17. Indices shift with the rules and the number of decks. For example, in many games where the dealer hits soft 17, basic strategy already doubles 11 against an ace.

The Fab 4 surrenders

If the game offers late surrender, four more deviations are worth knowing:

Which deviations to learn first

1. Insurance at +3. It comes up constantly at high counts, exactly when your biggest bets are out.
2. 16 vs 10 and 15 vs 10. Two of the most common stiff hands you'll face.
3. The rest of the list, a few at a time. Add new indices only once the previous ones are automatic.

A handful of deviations played perfectly is worth more than dozens played with hesitation. Be aware that some plays, like splitting tens, are well known to casino staff and can draw attention.