The basic rule
When you're allowed to double after splitting, split 2s and 3s against a dealer 2 to 7. Against an 8, 9, 10 or ace, just hit. The dealer's strong cards make it too risky to put twice the money on the table with two small starting cards.
Why splitting works against weak dealer cards
Against a 2 to 7, the dealer often has to draw and can bust. Each split hand starts with a 2 or a 3 and can easily grow into a decent total, and if you catch an 8 or a 9 you get a 10 or 11 that you can double. Those extra doubling chances are a big part of the value.
When the table doesn't allow doubling after splitting
If you can't double after a split, splitting loses a lot of its appeal. In that case, split 2s and 3s only against a dealer 4 to 7, and hit against 2 and 3.
Don't confuse them with 4s and 5s
A pair of 4s should only be split against a 5 or 6, and only if you can double after splitting. A pair of 5s should never be split: a total of 10 is a great hand to double.
FAQ
Should I split 2s against a dealer 8?
No. Against 8, 9, 10 or an ace, just hit.
Does doubling after split matter?
Yes. Without it, split 2s and 3s only against a dealer 4 to 7.
Related guides
Strategy and maths reduce the house edge, but never remove it. 18+, play responsibly.