A player can choose to split if they have two of the same cards, the amount of original bet is doubled and both cards are played as separate hands, player may choose to double again, if he receives the same as originals.
Aces: A pair of aces is probably the best hand you can get in blackjack, and splitting that hand is what makes it great. So you should always split a pair of aces. Before the split you have a mediocre hand of 12 and after the split you have two potential blackjacks, assuming of course, as always in basic strategy, that you will get tens.
2s: This one is played similarly as 3s and 7s. If the dealer has 2 to 7, then you should split, otherwise take a regular hit. Again we assume that the hole card is a 10, making the dealer go to 12 to 17, while we end up with a 12.
3s: Well same as before, if dealer has 2 to 7 split, otherwise hit. Again same reason, you assume you and dealer both get 10s, making your hand quite unfavorable, and especially if the dealer has an 8 or higher, in which case you should not invest more money into this game.
4s: Split only if the dealer has 5 or 6, in all other cases you have a potential hand of 18, which is a solid hand, but if the dealer has 5 or 6 you are counting on his busting, so after split you are hoping for pair of 14s and waiting for dealer to bust.
5s: This is one of the more famous blackjack hands, because most beginners make the mistake of splitting this hand, but a pair of 5s should never be split, because this is a potential of 20, a great hand and should never be thrown away, for a small chance of improvement.
6s: Well quite standard, if the dealer has 2 to 6 then split it, in all other cases just regular hit. In this case you are again relying on dealer bust, and by splitting you are hoping of getting some hands that will beat him even if he doesn’t.
7s: As mentioned before played same as the 3s. If the dealer has 2 to 7 then split, otherwise just take a hit. Reason for this is that assuming you both get 10s, the dealer ends up with either a stiff hand or 17, while you have also 17, so you aren’t loosing, while twice the chance of getting lucky and getting something better. In case the dealer has an 8 or better, you are probably going to lose anyway, so it would be a bad idea to invest more money, best option is to take the hit and just hope for the best.
8s: You should always split 8s, you go from one not very good hand, to two potentially great hands.
Splitting hands and when to split9s: In case the dealer has 2-6 you should split, because you get to play 2 potentially great hands versus dealer stiff hand, you win in most cases.
10s: This is a hand for standing, 20 is a great hand, in most cases it’s a winner, and with very small chance of actually improving it, it is just not worth to split it.