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Blackjack Deviations & the Illustrious 18

Deviations — also called index plays — are the points where a card counter should break from basic strategy because the true count has changed the odds. They are an advanced layer on top of counting, and a handful of them deliver almost all the value. This guide explains how deviations work, lists the famous Illustrious 18, and shows the single most profitable play.

What is a deviation?

A deviation is a decision where the count tells you to play a hand differently from basic strategy. Basic strategy is the mathematically best play averaged over a full, random shoe. But when counting tells you the remaining cards are unusually high or low, the best play for this moment can change.

Each deviation has an index number — the true count at or beyond which the play flips. The classic example is 16 versus a dealer 10. Basic strategy says hit. But its index is 0, meaning at a true count of 0 or higher you should stand, because the extra high cards make you more likely to bust if you hit.

The most valuable deviation: insurance at +3

Under basic strategy you never take insurance — it carries a house edge above 7%. But insurance is really a bet that the dealer’s hole card is a ten. When the true count is high, the deck is rich in tens, and that bet becomes profitable.

Take insurance at a true count of +3 or higher. This is the single most valuable index play in blackjack — for counters it is worth more than all the other deviations combined, because it comes up often and the swing is large.

The Illustrious 18

Popularized by Don Schlesinger, the Illustrious 18 are the eighteen highest-value deviations. Memorizing just these captures most of the gain available from deviating. The index is the true count at which you make the listed play:

HandIndex (true count)Play at/above index
Insurance+3Take insurance
16 vs 100Stand
15 vs 10+4Stand
10,10 vs 5+5Split
10,10 vs 6+4Split
10 vs 10+4Double
12 vs 3+2Stand
12 vs 2+3Stand
11 vs A+1Double
9 vs 2+1Double
10 vs A+4Double
9 vs 7+3Double
16 vs 9+5Stand
13 vs 2-1Stand (hit below)
12 vs 40Stand
12 vs 5-2Stand (hit below)
12 vs 6-1Stand (hit below)
13 vs 3-2Stand (hit below)

Indices are the widely published Hi-Lo values for multi-deck shoes and vary slightly by rule set. “Hit below” means the listed stand applies at the index and above; below it you follow basic strategy.

The Fab 4 surrenders

If your game offers late surrender, the Fab 4 add the most valuable surrender deviations on top of the Illustrious 18:

  • 14 vs 10 — surrender at true count +3 or higher
  • 15 vs 10 — surrender at +0 or higher
  • 15 vs 9 — surrender at +2 or higher
  • 15 vs A — surrender at +1 or higher

How much do deviations add?

Be realistic about the payoff. The bulk of a counter’s edge comes from betting more when the count is high, not from deviations. The full Illustrious 18 adds only about 0.1% to 0.2% to your edge. That is why the order matters: master accurate counting and a sound bet spread first, then layer in insurance at +3, then the rest of the Illustrious 18. A single counting error costs more than a deviation earns, so accuracy always comes first.

Frequently asked questions

What are deviations in blackjack?
Deviations, also called index plays, are situations where a card counter departs from basic strategy because the true count makes a different decision more profitable. For example, 16 versus a dealer 10 is normally a hit, but at a true count of 0 or higher you stand. Each deviation has an "index number" — the true count at which the correct play changes.
What is the Illustrious 18?
The Illustrious 18 is a list, popularized by Don Schlesinger, of the eighteen most valuable basic-strategy deviations for a card counter. They capture the large majority of the extra value available from deviating, so they are the ones worth memorizing first. Adding the "Fab 4" surrender deviations rounds out the most important plays.
What is the most valuable deviation?
Taking insurance at a true count of +3 or higher is the single most valuable deviation. Insurance is a bad bet under basic strategy, but when the deck is rich in ten-value cards the dealer is more likely to have a blackjack, and at +3 the bet turns profitable.
Do I need deviations to beat blackjack?
Most of a counter's edge comes from bet sizing, not deviations — deviations add roughly 0.1% to 0.2%. Learn accurate counting and a correct bet spread first. Add the Illustrious 18 only once your count is fast and error-free, because a counting mistake costs more than a deviation gains.