Double Down

Doubling down means doubling your original bet and receiving exactly one additional card. You cannot hit again after doubling. It's the most profitable play in many situations because you're increasing your bet when you have a mathematical advantage.

Basic strategy says to double on hard 11 against any dealer card 2-10, hard 10 against 2-9, hard 9 against 3-6, and various soft hands. These are the situations where you're statistically most likely to end up with a strong hand.

Failing to double when you should is one of the most costly mistakes in blackjack. You're leaving money on the table every time you miss an optimal double.