Common Spanish 21 mistakes
Spanish 21 punishes blackjack habits. These are the errors that cost the most, in order.
Short answer
The five expensive mistakes are: playing from a blackjack chart, never using double-down rescue, splitting suited sevens against a dealer 7, taking the Match the Dealer side bet, and sitting at a table where the dealer hits soft 17 without knowing it costs 0.40 percentage points.
1. Playing from a blackjack chart
This is the largest error, and the most common. A blackjack chart tells you to stand on hard 12 against a 4, 5 or 6, and to stand on hard 15 and 16 against a 7. In Spanish 21 all of those are hits, because the dealer busts about four percentage points less often with every weak upcard and because you bust less often on the draw. Learn the Spanish 21 chart or do not sit down.
2. Never rescuing
Double-down rescue is worth about 0.15 percentage points SIM and it is used on 1.40% of rounds. Players who have never heard of it simply give that back. Rescue a doubled total of 16 or less against a dealer 8, 9, ten-valued card or ace. Nothing else, and never against a 2 through 7.
3. Splitting suited sevens against a dealer 7
The chart says to split sevens against a 7, and for unsuited sevens that is right. If the two sevens share a suit, splitting throws away a live Super Bonus. The third suited seven arrives 1.40% of the time EXACT and pays a flat jackpot worth about 200 units at a minimum bet, which is roughly 2.8 units of expectation. Splitting cannot come close to that. Hit instead. Details on the bonuses page.
4. Taking Match the Dealer
The side bet carries a 3.055% house edge EXACT against the main game's 0.495%. It is about six times more expensive per dollar. See side bets.
5. Not reading the soft 17 rule
A dealer who hits soft 17 costs you 0.40 percentage points, taking the game from 0.495% to 0.893% SIM. That is the largest single rule variation you will see between two Spanish 21 tables in the same pit, and it is printed on the felt.
6. Not doubling on three or more cards
Spanish 21 lets you double at any point, on any total. A three-card 11 is a double. Players who only ever double on two cards are giving up about 0.17 percentage points SIM.
7. Chasing bonuses
Taking an extra card that the chart does not call for, in the hope of a five-card 21, loses more than the bonus pays. The chart already knows about the bonuses. It was computed with them in it.
8. Assuming the bonuses are always there
Bonus pay tables vary. If a table has cut the 5-card 21 from 3:2 or removed the 6-7-8 payout, the edge quoted on this site does not apply to that table. The full bonus set is worth 0.33 percentage points, most of the edge in the game.
Common questions
What is the single most costly habit?
Standing on stiff totals the way blackjack teaches. Hard 12 through 16 is played differently here, and those hands come up constantly.
Is it worth learning rescue if it comes up only 1.4% of the time?
Yes. Rare and large beats common and small. It is a fifth of the whole house edge, and it takes one line of the chart to learn.
How do I stop reverting to blackjack habits?
Practice against a grader that tells you the moment you are wrong. That is what the trainer is for.