Deviations

Basic strategy assumes a neutral shoe. When the count says the shoe is rich in tens, some of its plays are no longer best: you stand more often, because the dealer busts more often; you double more often, because your one card is more likely a ten; and you take insurance, because the dealer's hole card is more likely a ten. When the count is low, the reverse — you hit hands you would normally stand on.

The classic example is hard 16 against a 10. Basic strategy hits it, and loses either way most of the time. At a true count of 0 or higher you stand instead, because the ten that would bust you is now the likeliest card. The most valuable deviation of all is insurance at a true count of +3: insurance is a side bet that the dealer holds a blackjack, and it pays when more than a third of the remaining cards are tens.

Every deviation has an index: the true count at which the better play flips. At or above the index you make the deviation; below it you play basic strategy. Some indices are negative — those are plays you change when the count is low, such as hitting a 13 against a 2 at a true count of −1 or worse.

Deviations add roughly 10–20% to a counter's expected profit; the bet spread produces the rest. So learn them last, after the count and basic strategy are automatic, and learn the valuable ones first — the next chapter is that list.