Index Plays: When to Break Basic Strategy

Basic strategy tells you the best play for every hand. Index plays tell you the handful of moments when that is no longer true.

The apparent contradiction dissolves as soon as you look at what basic strategy is optimized for. It is computed for a full, freshly shuffled shoe: an average mix of high cards and low cards. A counter is not always looking at an average shoe. Halfway through a deck, after a run of small cards, the remaining cards can be far richer in tens and aces than the average, and a few decisions flip. An index number is the point at which they flip.

What an index number is

An index is a true count threshold. It says: at or above this true count, stop making the basic strategy play and make this other one instead. Below it, the basic strategy play stays correct.

Read one out loud and it becomes obvious. "Stand 16 versus 10 at 0 or higher" means that when you hold a hard 16 and the dealer shows a 10, you stand whenever the true count is zero or above, and you hit when it is negative. "Double 10 versus 10 at plus 4" means you take the extra bet only once the true count reaches plus 4.

The logic behind that particular pair is the logic behind all of them. A hard 16 against a 10 is a miserable hand either way, and the basic strategy answer (hit) is only slightly better than standing. Small cards improve your hit; tens bust it and also make the dealer's hidden card more likely to be strong already. When the shoe is rich enough in tens, the two options cross and standing takes over. The index is where the crossing happens.

Everything here is Hi-Lo and rule specific

Two warnings before any numbers appear, because both catch people out.

First, index numbers belong to a counting system. The values below are Hi-Lo true count indices. KO, Omega II and Zen assign different tags to the same cards, so their counts run on different scales and their published indices are different numbers. Using a Hi-Lo index while running a KO count is not an approximation, it is a mistake.

Second, index numbers shift with the house rules, particularly between a game where the dealer stands on soft 17 and one where the dealer hits it. Some published indices move by a point or more between the two. If your table's rules differ from the list you memorized, expect small discrepancies and use the list published for the game you are in.

Both warnings depend on getting the count itself right, which means the true count and not the running count. If that conversion is still fuzzy, read running count versus true count before going further, because every number on this page is a true count.

The examples worth knowing

These are canonical indices from Schlesinger's published list, the same set the Illustrious 18 draws on.

Two more from the same list round out the picture: split 10s against a 5 at plus 5, and split 10s against a 6 at plus 4. Those two are unusual because they break a rule most players treat as absolute (never split tens), and because they are conspicuous at a table in a way the others are not.

Notice the pattern. Most of the plays that turn on at a positive count involve standing on a stiff hand or putting more money out on a strong one. Both make sense when tens are abundant: the dealer is more likely to have a made hand or to bust off a stiff, and your double is more likely to land on a 10.

Insurance is the one to learn first

Insurance at plus 3 is the most valuable single deviation on Schlesinger's published list, and it is also the easiest to execute.

Insurance is a side bet that the dealer's hole card is a ten-value card, paying 2 to 1. Basic strategy always declines it, because in an average shoe there are not enough tens to make the price fair. But that is a statement about the average shoe. Once the remaining cards are rich enough in tens, the bet crosses into positive territory, and at a Hi-Lo true count of plus 3 or higher it is worth taking.

It earns its top spot for two reasons. The decision arrives every time a dealer shows an ace, which is often, and it is a clean yes or no with no card-drawing involved. There is no hand to misplay afterward. If you learn exactly one index number, learn this one.

The small handful that carries the value

Published index lists run far beyond the eighteen. Learning all of them is a project with steeply diminishing returns, which is exactly the observation behind the Illustrious 18: a short list of deviations that captures most of the available gain, because those hands come up often enough to matter and the plays are close enough that the count genuinely flips them.

A sensible order of operations looks like this:

• Insurance at plus 3, until it is automatic.
• 16 v 10 at 0 or higher, one of the highest-frequency playing deviations on the list.
• 15 v 10, 12 v 3 and 12 v 2, the other common stiff-hand flips.
• The doubles: 11 v A, 10 v 10, 9 v 2.
• Only then, the rarities such as splitting 10s against a 5 or 6.

Adding the twentieth index to your repertoire is worth a small fraction of what adding the first one was. Most of the gain is in the first few.

How to drill them

Index plays fail under time pressure in a specific way: the player holds the hand, remembers that a deviation exists, and then has to reconstruct the count and the threshold at the same moment the dealer is waiting. The fix is to separate the skills in practice and recombine them only when each is solid.

21 Trainer Pro splits the work accordingly. Single card and multi-card drills build raw counting speed. The true count drill trains the division by decks remaining. The index quiz asks for the deviation cold, so you know the thresholds without having to derive them. Index deviation training then covers the Illustrious 18, the Fab 4, extended indices and Ace-Five, and the bankroll simulator puts all of it into full shoes with a hand-by-hand mistake breakdown at the end. If your accuracy heat map shows the deviations breaking down on particular upcards, the Weakness Drill will target them. That per-upcard breakdown is the part worth watching, for the reasons covered in what a good strategy quiz measures.

None of that replaces the foundation. A deviation played on a miscounted shoe is worse than no deviation at all, because you have knowingly abandoned the play that was correct on average. Hi-Lo counting accuracy comes first, always.

The honest size of the prize

Index plays add a fraction of a percent for a counter who is already keeping an accurate count and sizing bets sensibly. That is a real gain in a game with thin margins, and it is also much smaller than most people assume when they first see the tables.

The larger truth around it stays unchanged. Counting gives a small long-run edge with large short-term swings, and the edge exists only when the count is right. Casinos are private businesses and may back off or refuse service to players they suspect of counting, and continuous shuffling machines make the whole exercise moot. No technique here removes risk, and none of it belongs anywhere near a real-money decision. Practice it as a skill, with virtual chips, and judge yourself on accuracy rather than results.

Frequently asked questions

What is an index number in blackjack?

An index number is the true count at or above which a different play becomes the correct one. Basic strategy assumes an average shoe, and the count tells you when the shoe is no longer average. If a play carries an index of plus 4, you make the basic strategy play below a true count of plus 4 and switch to the deviation at plus 4 and higher.

What does stand 16 v 10 at 0 or higher mean?

It means that with a hard 16 against a dealer 10, you stand whenever the Hi-Lo true count is zero or above, and hit when it is below zero. Per Schlesinger's published list this is one of the most frequently used deviations, because 16 against a 10 comes up often and the basic strategy play is close to a coin flip already.

Which index play is the most valuable?

Insurance at a true count of plus 3 is the single most valuable deviation on Schlesinger's published list. Basic strategy always declines insurance, but when the shoe is rich enough in tens the side bet turns in the player's favor. It matters because it comes up whenever the dealer shows an ace, and because the swing per decision is larger than most playing deviations.

Do index numbers change between counting systems?

Yes. Index numbers are tied to the specific count that produced them, so a Hi-Lo index is meaningless in KO, Omega II or Zen, which weight cards differently. Indices also differ between games where the dealer stands on soft 17 and games where the dealer hits it. Always use the index list published for the system and the rule set you are actually playing.

Are index deviations worth learning?

They add a fraction of a percent for a counter who already keeps an accurate true count, and a small handful of them carry most of that value. They are worth nothing at all without the count underneath, because a deviation made on a wrong count is simply a basic strategy error. Learn to count accurately first, then add insurance and a few high-frequency indices.

Keep reading

• The Illustrious 18, Explained
• Running Count vs True Count
• Hi-Lo Card Counting