There Are Times to Split 10s at Blackjack, but They're Rare

13 September 2004

By Alan Krigman

Anyone who's ever received advice, sought or otherwise, about blackjack is familiar with the edict, "never split 10s." And most have at least wondered why it should be so. Sure, a total of 20 is a strong pat hand. But a 10 is also a powerful starting point, strong enough for a double against anything other than 10 or ace. So why not split the pair and go for twice the money, especially when the dealer is vulnerable with something like a five or six?

One intuitive reason for this rule takes knowing that proper splits can be defensive or offensive. Defensive, when the total is an underdog but the components are either less so or bona fide favorites; eights versus a 10 is an example of the former, eights versus a seven or under illustrates the latter. Offensive, when the original hand has a positive expectation, but the separate elements have even higher statistical value; nines versus an eight or any up-card six or below are cases in point. Pairs of 10s meet neither of these criteria. A 20 is never an underdog, and separate 10s are never stronger than the combination.

A second intuitive explanation for the caveat against splitting 10s has to do with doubling down. A significant part of the potential offered by the split involves the possibility of a subsequent double. Say you start with a pair of sixes against a five. This is an offensive situation because your 12 is a theoretical loser while the individual sixes are projected to earn small profits. A two-card six, for instance a four-two, is a poor prospect, but the one-card six offers the hope of a three, four, five, or ace on which you could then double from the catbird seat. There is no draw to a one-card 10 that yields a hand on which doubling down is anything but disaster-in-waiting.

Inquiring minds, of course, will want something more tangible than mere intuition. Especially in the trompe l'oeil milieu of the casinos where nothing is as it seems. For individuals of this ilk, the accompanying table gives the average amounts solid citizens can expect to win or lose per $100 wagered at the start of a round when standing or splitting with pairs of 10s. Figures are for eight-deck games and -- for ace- or 10-up -- assume only playable hands (that is, dealers do not have blackjack).

Expected win or loss, standing or splitting
pairs of 10s against possible dealer upcards

upcard	stand	split
2	$63.84	$ 7.87
3	64.86	16.37
4	65.91	25.20
5	67.08	34.79
6	70.31	43.18
7	77.23	25.05
8	79.08	-0.37
9	75.66	-29.41
10	55.80	-44.02
A	65.48	-33.92

You could certainly luck out by splitting 10s even under the most adverse conditions. However, the table shows that the potential of splitting doesn't come close to that of standing regardless of how weak your gut tells you the dealer is. The best you can do is drop from a projected profit of $70.31 to $43.18 per $100 against a six. The worst is fall from an anticipated $65.48 gain to a theoretically forecast $33.92 loss against an ace.

Are there ever conditions under which splitting 10s is justified before the fat lady finishes singing? Yes, there are!

One that comes to mind is in tournament play, when winning a late round with the money you've bet by standing on a 20 will still not be enough to pass the leader, but twice as much will do the job. Another involves card counting. Were the "true count" at least +4 against a dealer's six or +5 against a five -- both of which imply a heavy proportion of 10s still in the shoe -- the expected value of splitting would exceed that of standing.

So, when you see players splitting their 10s, assume they're either clodhoppers or counters. Maybe both. If you're thinking about doing it yourself, and "thinking about" is the operative phrase, you're not a counter; you're probably someone the perceptive poet, Sumner A Ingmark, had in mind when he mused:

Woe betide those lacking fact,
Who leap where angels fear to act.