The whys of basic strategy

Seth is a friend of a friend, someone I've known casually for a couple of decades. He's the kind of guy who always wants to know "why." Why does car A get more miles to the gallon than car B? Why do bats fly at night? Why don't I like beets?

His latest "why" collection came up while he was learning basic strategy in blackjack. He worked at memorizing basic strategy charts and practiced on the computer, then peppered me with questions.

"I have to know," he said. "Why is basic strategy different for single-deck games? You seem to double down more with a single deck. And when you use multiple decks, why do you double on 11 against an ace when the dealer hits soft 17, but not when he stands on all 17s?"

Slow down, I told him. Let's take one question at a time.

"OK, first things first," he said. "Why do you double down more in single-deck games? The charts say to double down on 8 against a 5 or 6 in a single-deck game, but you don't double on 8 with multiple decks. And you double on 11 against an ace in single-deck games, but with six decks, you only double on 11 against an ace if the dealer hits soft 17. It's very confusing."

The big issue here is that each card played has a greater effect on the composition of the remaining deck when fewer decks are used. Let's say you have a 5 and a 6 for a total of 11, and the dealer has an ace. In a single-deck game, 16 of the remaining 49 cards — 32.7% — are 10 values that could give you a 21. When six decks are used, there are 309 cards other than your three knowns, and 96 — 31.1% — are 10 values.

You have a greater chance of getting that 10 in the single-deck game, so you double down more.

"And 8 vs. 5 or 6?"

You have an improved chance of making an 18 or better that can win even if the dealer doesn't bust. On top of that, the increased percentage of high cards makes the dealer more likely to bust with 5 or 6 when only a single deck is used.

"But why double on ace vs. 11 in a multi-deck game when the dealer hits soft 17, and not when he stands on all 17s?"

Because the dealer busts more often when he or she hits soft 17. If the dealer stands on all 17s, ace-6 and other soft 17s are a never-bust hand. Dealers who hit soft 17 won't bust all that often, but the times they do bust shift the percentages enough to change the double-down strategy.

"But I thought it was better for players if the dealer stood on all 17s," Seth said. "But you can double down more when the dealer hits soft 17. So why do the books say it's better if the dealer stands on all 17s?"

It is better overall for the dealer to stand on all 17s. In fact, it tacks an extra two-tenths of a percent onto the house edge when the dealer hits soft 17. You have a small gain in the 11 vs. ace double-down situation of the dealer hits soft 17, but you give back much more than that in other situations.

"Why?"

Because hitting soft 17 gives the dealer a chance to improve a pretty weak hand. A 17 will win for the dealer only if the player busts or stands with totals of 16 or less. If the dealer hits soft 17, then he or she improves the hand to a total that will win more often by drawing an Ace, 2, 3 or 4, and leaves a 17 that is no worse than the starting point by drawing a 10, Jack, Queen or King.

Hitting the soft 17 become problematic for the house with only 5 of 13 card denominations — 5, 6, 7, 8 and 9. Even with those draws, the dealer doesn't bust. He or she can just draw another card.

The bottom line is that the dealer will improve the hand often enough that it's a net gain for the house — and a net loss for players — if the dealer hits soft 17.

"OK, I guess I can see that. One more question before I let you go. Is it always best to follow basic strategy?"

It is for most players. There are exceptions for those who know about counting cards and composition-dependent strategies.

"I haven't really worked on any of that yet," Seth said. "I wanted to make sure I had basic strategy down. When I study the other things, I'm sure I'll have more questions."

I'll be here.