Doubling down: 9, 10, 11

Doubling lets you put more money out at the moment the matchup favors you. Declining that offer is one of the most expensive habits in blackjack. The big three are hard 9, 10, and 11, one-card draws that often finish 19–21 while the dealer is still at risk of busting.

11 doubles against everything, ace included, at H17 tables like our default. The dealer's extra bust risk from hitting soft 17 tips it. At S17 tables, 11 v ace is a hit. Any ten-value card makes your 21, and there are more of those in the deck than any other rank. 10 doubles against 2 through 9 and not against a 10 or ace. The doubled bet gets one card, and against a strong upcard the draws that leave you stuck on 13 with double the money out cost more than the good draws earn. 9 doubles only into the weak middle, 3 through 6, where dealer busts make up for the weaker totals a 9 draws to.

Hitting 11 against a 6 instead of doubling wins the hand just as often and wins half as much. The EV engine prices that at about $34 per $100 of forgone value. When the Checkpoint flags a missed double, you played a winning hand and got paid half.

This Checkpoint runs hard 8 through 11. Hard 8 never doubles at these rules, and you need to know where the region stops as well as where it starts. Soft doubles (A-2 through A-7) are in the soft-hands Lesson.