When playing the game of blackjack, if you want to reduce the house edge to a minuscule 0.5%, you have to follow basic strategy perfectly, which includes hitting, standing, doubling and splitting at the right times. In this series, we are covering the situations where you would double.
If you have a hand of ten, you should double down if the dealer has an up card of two through nine. That includes if you have a pair of fives, since you never split a pair of fives. If the dealer shows a ten or ace, you should hit.
One of the principles on which blackjack basic strategy is based is that there are more cards with a value of 10 (4/13) than any other value (1/13 each). For that reason, the dealer is more likely to have a ten in the hole than any other number and if you take a hit, you are more likely to receive a ten than any other number.
Taking that into account, if you have a ten, drawing another card gives you good odds of getting a 20. That hand can only be beaten by one hand, a 21. If the dealer shows a two through a nine, he has a good chance of having a hand of 12-19. All of those hands would be beaten if you had a 20. Not only that, but if the dealer has a 12-16, he has a good chance of busting. Therefore, if the dealer has any of those cards as up cards, the odds say that you will beat the dealer if you draw one more card. For that reason, it is good strategy to double your bet in that situation.
If the dealer has a ten or an ace as an up card, though, you are in trouble. Again, assuming the likelihood of the dealer having a ten in the hole, that means he likely has a hand of 20 or 21. If the dealer has a 21, you can’t win and can only tie if you also draw a 21. If the dealer has a 20, you can only beat him with a 21 of your own or tie with a 20. Since the odds are against you when the dealer has either of those hands, you do not want to put more money on the bet. For that reason, in this situation you would take a hit instead. You never stand with a hand of ten because there is no risk of busting.