Doubling down is the most fun move in blackjack and the one beginners get wrong most often - usually by not doing it enough. The instinct is to play it safe. But doubling isn't a gamble, it's the spot where the math says push more chips in, and skipping it leaves money on the table. Here are the rules, short enough to memorize.
You double your original bet and take exactly one more card - then you're done, no more hits. So you only want to do it when one card is likely to give you a strong total and the dealer is in trouble. That's the whole logic: more money in when you're ahead, not when you're hoping.
Soft doubles trip people up because the hand can't bust, so it feels like you should just hit. But against a weak dealer card, doubling a soft hand prints money:
The pattern under all of it: you double when the dealer is most likely to bust (showing 4, 5, or 6), and from a hand that one card can turn strong. Memorize "the dealer's weak cards are 4-5-6" and most of these fall into place.
To be honest, nobody memorizes a list like this by reading it once. The doubles are exactly the decisions that feel wrong in the moment - doubling a soft 17, pushing more money in when you're nervous. The fix is reps. The strategy drill throws random hands at you and grades every call against perfect play, and the full strategy chart shows every double in one grid. Twenty minutes and these stop feeling like guesses.
Random hands, instant feedback, graded against perfect basic strategy.
From strength against a weak dealer: always double hard 11, double hard 10 unless the dealer shows 10 or Ace, double hard 9 vs a dealer 3-6, plus the soft doubles vs the dealer's weak cards.
You double your bet and take exactly one more card, with no further hits. It's how you put more money down when the odds favor you.
Almost always — it can't bust and any 10 makes 21. The only close exception is vs a dealer Ace where the dealer hits soft 17.
At most tables yes (double-after-split). A few don't allow it; check the felt. It tweaks a few split decisions.