You hold a hard 10 (no ace counted as 11). The dealer shows a 6. Here is the mathematically correct basic-strategy play — and why.
| Double ← | +0.5756 |
| Hit | +0.2878 |
| Stand | -0.1537 |
| You bust on the next card | 0.0% |
| Dealer 6 busts | 42.3% |
Double is not close: it beats Hit by 0.2878 per unit. Getting this one wrong costs $2.88 on a $10 bet, every single time. Across the 340 situations in this chart it ranks 36th by expected value — better than 304 of them, worse than 35. Expected value is per unit staked, computed for multi-deck, dealer stands on soft 17, double after split allowed. The model and the code are published as data.
Hard 10 is a premium doubling hand. One card is very likely to give you a strong total, and against a 6 (which busts 42% of the time) you want maximum money on the table while you have the edge. Doubling wins you more in exactly the spots where you're a favourite.
If doubling isn't allowed (e.g. after hitting), simply hit instead — the extra card logic stays the same, you just can't add the extra bet.