Blackjack Deviations: The Illustrious 18 & Fab 4
Deviations are count-triggered exceptions to basic strategy. The famous Illustrious 18 + Fab 4 capture roughly 80-90% of all deviation value — and taking insurance at true count +3 is worth more than the rest combined.
Basic strategy is the best play for a neutral shoe. But a card counter knows the shoe is rarely neutral — and when it's rich in tens, a handful of "wrong" plays become right. These are deviations, and there's a strict prerequisite: they are only for counters. Without a count, every one of them is a strictly losing play. Learn basic strategy first, then counting, and only then this page. Done in that order, deviations add roughly 20-40% to a counter's edge.
How index numbers work
Each deviation comes with an index number: make the deviation when the Hi-Lo true count is at or above the index. A few indices are negative — those work in reverse, telling you to abandon a basic-strategy stand and hit when the count drops below the index. Example: 16 vs 10 has an index of 0, so you stand at true count 0 or higher and hit below it. 13 vs 2 has an index of −1, so you keep standing down to −1 and hit only when the count falls below that.
The Illustrious 18 (multi-deck Hi-Lo)
Don Schlesinger's ranking of the 18 most valuable deviations, in rough order of value. These are the standard published multi-deck Hi-Lo indices — exact rules and counting system shift them by ±1, which is fine; deviation value is forgiving near the index.
| Your hand | Dealer shows | Deviation | Index |
|---|---|---|---|
| Insurance | A | Take insurance — the single most valuable deviation | +3 |
| Hard 16 | 10 | Stand (basic says hit) | 0 |
| Hard 15 | 10 | Stand | +4 |
| 10,10 | 5 | Split (see cover note below) | +5 |
| 10,10 | 6 | Split (see cover note below) | +4 |
| Hard 10 | 10 | Double | +4 |
| Hard 12 | 3 | Stand | +2 |
| Hard 12 | 2 | Stand | +3 |
| Hard 11 | A | Double (S17 tables — H17 basic already doubles) | +1 |
| Hard 9 | 2 | Double | +1 |
| Hard 10 | A | Double | +4 |
| Hard 9 | 7 | Double | +3 |
| Hard 16 | 9 | Stand | +5 |
| Hard 13 | 2 | Stand (hit below −1) | −1 |
| Hard 12 | 4 | Stand (hit below 0) | 0 |
| Hard 12 | 5 | Stand (hit below −2) | −2 |
| Hard 12 | 6 | Stand (hit below −1) | −1 |
| Hard 13 | 3 | Stand (hit below −2) | −2 |
Insurance at +3 deserves its top billing: it comes up on every dealer ace, and at +3 the shoe is ten-rich enough that the 2:1 payout becomes a positive bet — at exactly the moments your wager is biggest. The full normal-game case against insurance is here.
The Fab 4 (surrender deviations)
Four extra indices for tables offering late surrender. Basic strategy already surrenders 15 vs 10 and 16 vs 9/10/A; the Fab 4 adds count-triggered surrenders on hands basic strategy plays out.
| Your hand | Dealer shows | Deviation | Index |
|---|---|---|---|
| Hard 14 | 10 | Surrender | +3 |
| Hard 15 | 10 | Surrender | 0 |
| Hard 15 | 9 | Surrender | +2 |
| Hard 15 | A | Surrender | +1 |
Why deviations work
A high true count means the remaining shoe is dense with tens and aces. That shifts two numbers at once: the dealer's bust chance on stiff up-cards rises, and your hit cards get more dangerous. Plays that were nearly coin flips at a neutral count flip outright — 16 vs 10 is the textbook case, sitting so close to even at TC 0 that a single point of count decides hit versus stand. The same ten-density makes doubling 10 and 11 stronger and insurance profitable.
A risk-averse note on splitting tens: breaking up a made 20 screams "card counter" to any pit boss watching, which is why many professionals skip both ten-split entries for cover. The EV given up is small; the longevity gained is not.
Frequently asked questions
Should beginners learn deviations?
No. Deviations are worthless — actually harmful — without an accurate count behind them. Perfect basic strategy and flawless counting accuracy are each worth far more. Deviations are the last 20-40% of polish on an edge you must already have.
How many deviations do I actually need?
Insurance at +3, 16 vs 10 at 0, and 15 vs 10 at +4 alone capture a large share of total deviation value. Many successful counters play just those three plus the Fab 4 and call it a career.
Do deviations change with H17 vs S17?
A few indices shift by about a point. The famous example is 11 vs ace at +1 — an S17-table deviation, since H17 basic strategy already doubles it. The headline indices (insurance +3, 16 vs 10 at 0) hold across rule sets.