Very little happens at the blackjack table that surprises me. I’ve been playing a very long time, and I take other players’ strategic idiosyncracies in stride.

Still, it gave me pause in early April when a 30-ish fellow at my table was dealt a pair of 5s and the dealer turned up a 6 in a six-deck game in which the dealer hit soft 17, double downs were permitted after any first two cards and pairs could be split up to three times. The player pushed out a second bet, and the dealer asked, “One card?”

So far, that’s standard stuff. When I make the play, I automatically hold up an index finger and say one card, so the dealer knows I’m going to double down, and not split the pair.

But this time, the player said, “No. Split ’em.”

He did it so automatically and smoothly, it was obvious he’d played the game before. This was no beginner hesitating over basic strategy play. But it was a bad play nonetheless, and the table gave a collective, “Ooooh.”

I held my tongue. I never give advice at the tables unless asked. Others weren’t so reticent.

The dealer started with a mild, “Are you sure, honey?” The middle-aged man at third base was more emphatic. “You really don’t want to do that,” he lectured. “There’s no way you want to break up a 10.”

The response was a curt, “Yes I do.” And he did.

Basic strategy tells us that dealt 5-5, the best play is to double down if the dealer’s face up card is 2 through 9, or to hit if the dealer shows a 10-value or an Ace.

There are two things in play. The first is that 10 is a strong starting hand. Starting with 10 against a dealer’s 6, we have an edge. That’s why basic strategy calls for us to double down, so that we can get more money on the table and maximize profit potential. According to the expected returns table at WizardofOdds.com, you’ll average 29 cents in profit per $1 of your original wager if you hit 5-5 vs. 6, and push that profit to 59 cents by doubling down.

If you split, 5 is a much weaker starting point to the hand. Starting with 5 vs. 6 still is a profitable hand for the player because you can stop hitting if the next couple of cards leave you with a weak hand, while the dealer must hit until either getting 17 through 21 or busting. But the profits are much smaller if you double down, or even if you hit. When you split 5-5 vs. 6, the average profit dives to 10 cents per $1 of your original wager.

The numbers leave no doubt: Splitting 5s costs you money. By far the most profitable play when you’re dealt 5-5 and the dealer shows a 6 is to double down. The next best play is to hit with no additional bet, and the weakest move is to split the pair.

What actually happened on that April morning was a bit of an anticlimax. The splitter won on one hand and lost on the other. I didn’t track the cards to see what then result would have been if he had gone the one-card route.

Still, the best bet is always to play the odds. You’ll win some and lose some, but overall it’ll help keep your bankroll healthier if you refrain from splitting the 5s.

Gambling author and columnist John Grochowski’s weekly newspaper column began at the Chicago Sun-Times and is now syndicated nationally. He also regularly makes TV and radio appearances about gambling. His column appears weekly.