Doubling 9, 10, 11

Doubling down is putting out a second bet equal to your first, in exchange for taking exactly one more card. It's one of the most profitable plays in blackjack — but only when used correctly. The rule of thumb: double when you're already favored. - Hard 11: double vs 2–10 always. Add A only under H17 (dealer hits soft 17 — gives the double back enough to be +EV). In S17, hit 11 vs A — the dealer's stand on soft 17 closes the double's recoupment window. - Hard 10: double vs 2–9. Don't double vs 10 or A — those are too strong. - Hard 9: double vs 3–6 only. That's the narrow weak-dealer window. Doubling on hands you're NOT favored on is a classic leak. Never double 10 vs dealer 10.