One of the key concepts in blackjack doubling strategy is the "ten factor". There are sixteen cards that have a value of 10 as per blackjack rules and hence by taking an additional card the hand value is most likely to increase by 10.
The blackjack doubling strategy depends on whether the playerís hand gives a hard total or a soft total. In blackjack a soft total is one in which the ace takes the value of 11, for example an ace and a five total to a soft 16. All other totals are hard totals. The blackjack doubling strategy for hard totals is discussed first. If the player has a hard 11 then he should always double because the ten factor will give him a very strong hand. The playerís hard 10 is a slightly weaker blackjack total and the player should not double if the dealer holds a 10 value card or an ace, because in that case the ten factor gives the dealer the advantage. If the player has a hard 9 he should double only when the dealer holds a stiff card. A stiff card is a blackjack term for the dealerís cards that have a high chance of busting. Cards from two to six are stiff cards. The playerís hard 8 becomes even weaker and doubling is restricted to the weaker of the stiff cards, which are fives and sixes. A player should not double with a hard total of 7 or less because the only one more card blackjack doubling rule will restrict the playerís hand value to 17.
Playerís soft totals focus on the dealerís busting likelihood. If the player has an ace with the second card being from a two to a seven the player should double only when the dealer holds a stiff card. If the player holds an ace and an eight or nine he is in a strong position and can only weaken his hand value by taking another card. Finally if the player holds a blackjack he has already won.
This blackjack doubling strategy applies to single deck games played under normal rules. If the blackjack game uses multiple decks or is being played under Nevada rules or the no hole card rule then there will be minor variations to the strategy.