What are deviations (index plays) in blackjack?

Deviations are departures from basic strategy that become correct when the card count reaches a specific True Count threshold, called an index number. Basic strategy assumes a neutral deck; when the count says the remaining shoe is rich or poor in high cards, a handful of borderline decisions flip.

What is the Illustrious 18?

The Illustrious 18 is Don Schlesinger's list of the eighteen most valuable Hi-Lo deviations, published in Blackjack Attack. These eighteen plays capture roughly 80-90% of all the value available from playing deviations, which is why most counters memorize only these plus the Fab 4 surrenders.

How do I use an index number?

Compare the True Count to the index: if the True Count is equal to or higher than the index, make the deviation play; otherwise play basic strategy. For negative indices (like 13 vs 2 at -1), you deviate when the count falls BELOW the index.

Should I learn deviations before card counting?

No. The order is: perfect basic strategy first, then Hi-Lo counting with flawless True Count conversion, then bet spreading, and only then playing deviations. Deviations add roughly 20% to a counter's edge - betting correctly adds far more.

Deviation Charts: The Illustrious 18 & Fab 4

Basic strategy is only "perfect" for a neutral shoe. Once you're counting, the composition of the remaining cards keeps changing - and at certain counts, a handful of borderline plays flip. These deviations (or index plays) are how card counters squeeze out the last chunk of edge, and they're the difference between someone who counts and someone who plays like a pro.

How to Read an Index Number

The rule: deviate at the index, or above it

Each play has an index - a True Count threshold. If the True Count is at or above the index, make the deviation. Otherwise, play basic strategy. Example: 16 vs 10 has an index of 0, so at a true count of 0 or higher you stand; below 0 you hit. Negative indices work in reverse: 13 vs 2 has an index of -1, meaning you abandon your normal stand and hit when the count drops below -1.

Indices below assume 6 decks, dealer stands on soft 17, Hi-Lo count. H17 games shift a few indices by about a point - noted where it matters. If you're still shaky on True Count conversion, practice with our True Count drill first; deviations are useless with a wrong count.

The Illustrious 18

Ranked roughly by value. The first few entries are worth far more than the rest - if you only memorize five, make them Insurance, 16 vs 10, 15 vs 10, and the two 12 vs low-card stands.

Your Hand	Dealer	Index (TC)	Deviation	Basic Strategy Says
Insurance	Ace	+3	Take Insurance	Never insure
16	10	0+	Stand	Hit (surrender if offered)
15	10	+4	Stand	Hit (surrender if offered)
10,10	5	+5	Split	Stand
10,10	6	+4	Split	Stand
10	10	+4	Double	Hit
12	3	+2	Stand	Hit
12	2	+3	Stand	Hit
11	Ace	+1	Double	Hit (S17)
9	2	+1	Double	Hit
10	Ace	+4	Double	Hit
9	7	+3	Double	Hit
16	9	+5	Stand	Hit (surrender if offered)
13	2	-1	Hit below -1	Stand
12	4	0	Hit below 0	Stand
12	5	-2	Hit below -2	Stand
12	6	-1	Hit below -1	Stand
13	3	-2	Hit below -2	Stand

Insurance vs Ace: The single most valuable deviation - worth more than the other 17 combined for most bet spreads.
16 vs 10: The most frequently used index. At a true count of 0 or higher, stand.
10,10 vs 5: Massive attention magnet. Many pros skip the ten-splits for camouflage.
11 vs Ace: In 4-8 deck S17 games basic strategy hits 11 vs Ace; double at True Count +1 or higher, hit below. In H17 games (and single or double deck) basic strategy already doubles.
13 vs 2: A negative index: in bad counts you HIT a hand you'd normally stand on.

The Fab 4 (Surrender Deviations)

Four surrender plays that only apply where late surrender is offered. Combined with the Illustrious 18, these cover essentially all the playing-deviation value a Hi-Lo counter can get.

Your Hand	Dealer	Index (TC)	Deviation	Basic Strategy Says
15	10	0+	Surrender	Surrender
15	9	+2	Surrender	Hit
15	Ace	+1	Surrender	Hit (S17)
14	10	+3	Surrender	Hit

15 vs 10: Below 0, take the hit instead of surrendering.
15 vs Ace: In H17 games basic strategy already surrenders; the index to keep surrendering is roughly -1.

How Much Are Deviations Worth?

Roughly 20% of a card counter's total edge comes from playing deviations - the rest comes from bet spreading. That's why the learning order matters: perfect basic strategy, then the Hi-Lo count, then betting, and deviations last. A counter who plays perfect basic strategy with a good spread but zero deviations still has most of the available edge; a player who fumbles the count while attempting index plays has none.

Also worth knowing: the deviations are where the camouflage risk lives. Nobody notices a hit on 16 vs 10 - but splitting tens against a 6 makes the pit boss put down their coffee. Weigh each play's value against the attention it draws.

Common Questions

What are deviations (index plays) in blackjack?
Deviations are departures from basic strategy that become correct when the card count reaches a specific True Count threshold, called an index number. Basic strategy assumes a neutral deck; when the count says the remaining shoe is rich or poor in high cards, a handful of borderline decisions flip.
What is the Illustrious 18?
The Illustrious 18 is Don Schlesinger's list of the eighteen most valuable Hi-Lo deviations, published in Blackjack Attack. These eighteen plays capture roughly 80-90% of all the value available from playing deviations, which is why most counters memorize only these plus the Fab 4 surrenders.
How do I use an index number?
Compare the True Count to the index: if the True Count is equal to or higher than the index, make the deviation play; otherwise play basic strategy. For negative indices (like 13 vs 2 at -1), you deviate when the count falls BELOW the index.
Should I learn deviations before card counting?
No. The order is: perfect basic strategy first, then Hi-Lo counting with flawless True Count conversion, then bet spreading, and only then playing deviations. Deviations add roughly 20% to a counter's edge - betting correctly adds far more.

Count Through a Real Shoe

Deviations only work with an accurate True Count. Our trainer deals from a continuous 6-deck shoe - no reshuffling between hands - so you can practice keeping the count exactly like at a real table.

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