For a start, never split a pair of face cards or a pair of tens! With these cards you have a very strong hand of 20 and there is no point in ruining it by splitting. By contrast, always split a pair of aces. A single ace with another card is a much stronger hand than two aces and by splitting your chances of finishing with a high hand or even 21 are very good.
A pair of 9s are best split if the dealer is showing a weak card of 2 to 6. If the dealer is showing an 8 you should also. Likewise, if the dealer is showing a 9 you should split because if the dealer has gets 19 it will beat your current hand of 18. If the dealer is showing a 7 then you should stand. Always split 8s even if the dealer is showing an ace. Your hand now stands at 16, which is very likely loser. You are better off taking a chance and splitting.
A pair of 7s should be split if the dealer's upcard is 7 or less. A hand of 14 is a weak hand but by splitting there is a good chance you will convert that into a pair of 17s. If the dealer is showing an 8 there is a good stay with what you've got and be prepared to lose the hand. You are better off just losing the one hand against an 18 than losing two hands of 17 against a dealer's 18. Split a pair of 6s if the dealer is showing a 6 or less. It is risky to split 6s because you might end up with two hands of 16 so only do it if the dealer is showing a weak hand of below 6 that may lead to the dealer busting.
Never ever split a pair of fives. Your hand is now a very strong ten. Keep the fives and take a hit. Do not split fours either unless the dealer is showing a 5 or 6. If you split a pair of 4s you are likely to hit a difficult 14. 5 or 6 for the dealer however is likely to give the dealer a difficult 15 or 16 so it is worth your while splitting.
You should split 2s or 3s if the dealer is showing a weak upcard of below 7.
By knowing when to split and when to take a hit with what you've got you can add decisive strategy to your Blackjack play.
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