Basic strategy is the foundation of winning blackjack play, but a card counter eventually reaches a point where the correct decision is no longer fixed. When the remaining cards become unusually rich in high or low cards, a few close basic-strategy decisions can flip.
Those count-dependent changes are called strategy deviations, and they are one of the skills that separate simply keeping a Hi-Lo count from actually using the information it provides.
The good news is that you do not need to memorize hundreds of obscure exceptions. A small group of high-value plays captures most of the practical benefit. The best-known set is Don Schlesinger’s Illustrious 18, often paired with four important surrender deviations known as the Fab 4.
What is a blackjack strategy deviation?
A strategy deviation is a play that changes from basic strategy because the count has crossed a specific threshold.
Basic strategy assumes a particular game and the normal distribution of unseen cards. Card counting adds information about the composition of the cards that remain. When enough low cards have already appeared, for example, the remaining shoe becomes relatively richer in 10s and aces. That can make standing, doubling, splitting, surrendering, or taking insurance more attractive in certain situations.
The threshold where the preferred play changes is called an index number.
For Hi-Lo, these decisions are normally based on the true count rather than the running count. If an index for a play is +4, you make the deviation when your true count reaches the specified threshold under the index convention you are using.
Why deviations work
Hi-Lo assigns +1 to 2–6, 0 to 7–9, and −1 to 10s and aces. A positive count therefore indicates that proportionally more high cards remain than normal.
That changes several pieces of blackjack math. More 10s can improve double-down opportunities, make standing on some stiff hands relatively stronger, increase the chance that the dealer has blackjack behind an Ace, and occasionally make splitting a strong made hand worthwhile.
This does not mean that every positive count calls for aggressive play. Each decision has its own crossover point. A true count of +1 might change one hand while another does not change until +5.
What are the Illustrious 18?
The Illustrious 18 are a compact set of especially valuable Hi-Lo strategy indices popularized by Don Schlesinger in Blackjack Attack. Wizard of Odds reproduces the set with permission and reports that, in a six-deck game, learning these plays captures roughly 80%–85% of the value of learning every index number.
That makes the list useful for training: instead of trying to memorize an enormous deviation chart immediately, a counter can concentrate on the decisions that matter most.
| Play | Hi-Lo index |
|---|---|
| Insurance | +3 |
| 16 vs 10 | 0 |
| 15 vs 10 | +4 |
| 10-10 vs 5 | +5 |
| 10-10 vs 6 | +4 |
| 10 vs 10 | +4 |
| 12 vs 3 | +2 |
| 12 vs 2 | +3 |
| 11 vs Ace | +1 |
| 9 vs 2 | +1 |
| 10 vs Ace | +4 |
| 9 vs 7 | +3 |
| 16 vs 9 | +5 |
| 13 vs 2 | −1 |
| 12 vs 4 | 0 |
| 12 vs 5 | −2 |
| 12 vs 6 | −1 |
| 13 vs 3 | −2 |
Important: index tables are not universal laws. They depend on the counting system, rules, deck count, true-count conversion and rounding convention used to generate them. The table above follows the six-deck Hi-Lo treatment published by Wizard of Odds. Use indices matched to the exact system and game you have trained.
The three deviations to understand first
Insurance at +3
Insurance is normally a poor wager for a non-counter. That is why ordinary blackjack insurance strategy says to decline it.
Counting changes the question. Insurance pays 2:1 and becomes worthwhile when the unseen cards contain a sufficiently high proportion of 10-value cards. In the cited Hi-Lo system, the Illustrious 18 insurance index is +3.
This is also a useful lesson in what deviations really mean: you are not taking insurance because you “feel” that a 10 is coming. You are responding to a measured change in the composition of the remaining cards.
16 vs 10 at 0
Hard 16 against a dealer 10 is already a very close, bad situation. Standard strategy usually hits when surrender is unavailable, but the decision can flip quickly as the count rises.
The Illustrious 18 index is 0: at the index threshold, the high-card-rich remainder makes standing preferable under the specified system. This is one reason 16 vs 10 is among the most famous card-counting deviations.
Do not confuse this with late surrender. If surrender is available, the decision tree can be different; see the full 16 vs 10 strategy guide.
15 vs 10 at +4
Hard 15 against 10 is another ugly hand, but its crossover occurs later. In the Illustrious 18 table, you continue hitting below +4 and stand at +4 or above when surrender is not the preferred available option.
This example shows why “positive count means stand on stiff hands” is too crude. The magnitude of the count matters.
Why would a counter ever split 10s?
Basic strategy says do not split 10s. A total of 20 is exceptionally strong, so breaking it apart is normally costly.
At very high counts, however, the expected value of creating two hands against a weak dealer can eventually exceed the value of keeping the 20. The Illustrious 18 lists 10-10 vs 5 at +5 and 10-10 vs 6 at +4.
These are mathematically interesting deviations, but they are not beginner priorities. They require a substantial count and turn an unusual basic-strategy violation into a very visible play. A new counter should first master accurate counting, true-count conversion, betting and the higher-priority deviations.
The Fab 4 surrender deviations
If late surrender is offered, four additional count-dependent decisions are especially valuable:
| Hand | Hi-Lo surrender index |
|---|---|
| 14 vs 10 | +3 |
| 15 vs 10 | 0 |
| 15 vs 9 | +2 |
| 15 vs Ace | +1 |
At or above the applicable index, surrender becomes the preferred choice in the published table. These plays only matter when the table actually offers the relevant form of surrender, so first understand when blackjack surrender is available and how it works.
Deviations are not the same as changing your bet
Card counting has two separate applications:
- Betting decisions: changing wager size as the player’s estimated advantage changes.
- Playing decisions: changing hit, stand, double, split, insurance or surrender decisions at an index.
Strategy deviations add value, but they are not the main reason card counting can overcome the house edge. Betting more when the remaining shoe is favorable is a separate and crucial part of advantage play.
Wizard of Odds simulations illustrate the distinction. In its six-deck model, using the Illustrious 18 plus Fab 4 with different bet spreads and penetration levels produces different overall advantages. Better deck penetration also materially improves the opportunity because more cards are dealt before the shuffle.
How to practice deviations without wrecking basic strategy
The biggest training mistake is learning deviations before basic strategy is automatic. A deviation is an exception to a rule; if the underlying rule is shaky, adding exceptions creates confusion.
A better progression is:
- Master the correct basic-strategy chart for your target rules.
- Keep an error-free running count while cards are dealt.
- Convert accurately to true count using a realistic estimate of decks remaining.
- Add the highest-value indices a few at a time.
- Practice mixed drills where most hands still use ordinary basic strategy.
If true-count conversion is still slow, work on that before memorizing a long index table. The basic-strategy memorization method also works well for deviations: use flashcards, isolate errors, and practice boundaries rather than rereading a chart.
Common mistakes with card-counting indices
Using running count instead of true count. A running +6 with six decks remaining is very different from +6 with one deck remaining.
Mixing index tables. Numbers generated for different systems, rules or true-count conventions should not be casually combined.
Ignoring the table rules. H17/S17, surrender, deck count and hole-card procedure can affect the underlying strategy. A deviation cannot be applied correctly if you have identified the game incorrectly.
Learning rare deviations before core skills. Perfectly recalling an obscure index is not useful if you lose the running count or estimate the remaining decks poorly.
Treating an index as a prediction. A high count does not tell you the next card. It changes probabilities over the remaining population of cards.
How much of the full index table do you need?
For most people learning Hi-Lo, not much at first. The Illustrious 18 were designed precisely because the value of deviations is concentrated in a relatively small number of plays. Wizard of Odds reports that the set captures roughly 80%–85% of the value of learning all indices in a six-deck game.
That makes a sensible learning order: basic strategy first, counting accuracy second, true-count conversion third, then the highest-value deviations. Only after those skills are automatic does expanding into a full index set become a good use of practice time.
FAQ
What does an index number mean in blackjack?
An index is the count threshold at which a count-dependent play changes from its normal basic-strategy action. Hi-Lo indices are generally applied using the true count, subject to the convention used by that particular index table.
What are the most important blackjack deviations?
The Illustrious 18 are a widely used set of high-value Hi-Lo deviations. Insurance at +3, 16 vs 10 at 0, and 15 vs 10 at +4 are among the highest-ranked plays in the published six-deck list.
Should a beginner learn the Illustrious 18 immediately?
No. Learn basic strategy, running count, true-count conversion and deck estimation first. Deviations depend on all of those skills being accurate.
Do blackjack deviations work with a continuous shuffling machine?
Traditional shoe-based card counting becomes impractical when used cards are continually returned to the mix. Read our guide to continuous shuffling machines for the distinction between a CSM and an automatic batch shuffler.
Are the Illustrious 18 indices universal?
No. The familiar list is tied to Hi-Lo and specific assumptions and conventions. Different counting systems, rules, deck counts and conversion methods can produce different index numbers. Train from one internally consistent source for the game you intend to play.
Bottom line
Strategy deviations are where card counting starts influencing the way you actually play a hand. They are not guesses and they do not replace basic strategy. They are mathematically defined exceptions triggered when the true count shows that the remaining cards have shifted enough to change the best decision.
You do not need every possible index to begin. Master the fundamentals, learn the highest-value deviations, and expand only when you can execute the core system without hesitation. That approach captures most of the practical value while keeping the training load manageable.
