I was playing blackjack in Nevada, and the gentleman sitting at third base, all the way to the players’ left, had a basic strategy card in his shirt pocket. He didn’t use it on every hand, but every so often, he’d check it before he played.

The dealer mostly looked amused. The player wasn’t really slowing the game. Since was last to play, he checked the card while others were finishing their hands.

The dealer even teased him after one strategy check. She smiled and asked, “I don’t know, are you SURE about that one?” The player was deadly earnest, not realizing he was being teased. He pulled out the card again, and showed her the play. “See? That’s what it says.”

A short time after I joined the game, I was dealt a 7 and a 4. The dealer had an Ace face up, and after she looked at her down card to find she didn’t have blackjack, I doubled down.

The fellow at third base looked surprised. “I’m not so sure you should do that,” he said. “Do you want me to check the card for you?”

I told him no thanks, I was good on my double. I offered no further explanation, but there was basic strategy method behind what he saw as my madness. Like most low-limit games nowadays — and many high-limit games, too — this one had the dealer hit soft 17. Six decks were in play, and players were allowed to double down on any first two cards, including doubling after splitting pairs. We could split a pair up to three times, for a total of four hands. Aces could be split only once. No surrender was offered.

That’s a pretty run-of-the-mill game, with a house edge of 0.62 percent against a basic strategy player.

The “hit soft 17” provision is a basic strategy key. Overall, it’s better for the player if the dealer stands on all 17s. A 17 can’t beat you unless you bust. If you have a 17 or better, you don’t want the dealer to have the chance to draw a hand that will beat you.

However, there are a couple of close-call situations where the dealer hitting soft 17 opens double-down opportunities. An 11 vs. a dealer’s Ace is one of them. If I double and draw a Ace, 2, 3, 4 or 5, I lose if the dealer stands on all 17s. If the dealer hits soft 17, I can still win if the dealer busts.

So basic strategy draws a line. We double down on 11 if the dealer’s up card is 2 through 10 in all multideck games. We also double against an Ace of the dealer hits soft 17.

This time, it worked, though the 17 rule didn’t come into play. I drew a Queen for 21, and the dealer turned up a 9 for 20. I won my double.

The third baseman grinned and said, “I guess it was the right play this time.” I responded, “Yep, this time it worked.”

I had a pretty good idea what was going on with his strategy card, and why it told him I was making a bad play. My thoughts were confirmed a little while later when he was dealt an Ace and a 7 — a soft 18. The dealer’s up card was a 2.

“Gonna have to check on that one,” he said. He consulted the card, and decided to stand. The dealer looked puzzled, and one of the other players who pretty clearly knew his basic strategy caught my eye and shook his head. I nodded back.

The man on third caught the exchange. “Do you two doubt it?” he asked. “Do you want to check the card?” And he asked the dealer, “Can I show them?”

Before she could answer, the other basic strategy player said, “You might want to turn that card over.”

He had it sussed out. The card had basic strategy for the dealer standing on all 17s on one side, and strategy for hit-soft-17 games on the other. When our third baseman checked side two, he found it told him to double down on Ace-7 vs. a dealer’s 2, as opposed to standing on the hand if the dealer stands on all 17s.

There’s one more double-down difference that didn’t occur during our session. With Ace-8, basic strategy calls for us to double if the dealer hits soft 17, but stand if she doesn’t.

The dealer asked if the man wanted to change his play, and do what the card’s flip side said.

“No,” he said, “I made the decision. Next time I’ll know.”

The result? The dealer turned up a 2, a 3, a Jack, then another 2 for an 18. They pushed, and his lesson was free.

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. Look for John Grochowski at www.casinoanswerman.com, on Facebook (http://tinyurl.com/7lzdt44) and Twitter (@GrochowskiJ).