After you’ve been dealt your first two cards in blackjack, you have an option of doubling your initial bet for one additional card. There are 16 different cards with a value of 10 so it's best to do this when your first two cards equal 10 or 11.
It's also best to do this when the dealer is showing a 5 or 6 as the odds are in your favor here.
It's also best to do this when the dealer is showing a 5 or 6 as the odds are in your favor here.