Why double exists
Doubling lets you win more on strong starting totals (9, 10, 11 and some soft hands) when the dealer is weak. You get exactly one more card.
Common doubles
Hard 11 vs almost anything. Hard 10 vs 2–9. Hard 9 vs 3–6. Soft hands: double soft 13–18 only vs the upcards the chart lists. If you cannot double (not enough bankroll or wrong stage), take the fallback: usually hit (or stand for “Ds” soft plays).
If the chart says Hit, hit — even when it feels scary.
Tip: Never double a stiff hard 12–16. Those hands need flexibility, not one forced card.