Deviations and Index Plays
Short answer
Basic strategy is the best play when you know nothing about the remaining deck. Once you have a count, a handful of plays change. An index number is the true count at which the correct play flips from the basic-strategy move to the deviation.
Insurance is the single most valuable deviation
Insurance is a losing bet at a neutral count, but in a ten-rich shoe it becomes profitable. The standard rule is to take insurance when the true count is at or above its index, most commonly cited around +3. This one deviation is often worth more than all the rest combined, because it lets you make a profitable side bet exactly when you already have a large main bet out.
The Illustrious 18
These are the eighteen most valuable index plays: the deviations that earn the most given how often they come up. Each is a hand plus a dealer upcard plus an index number. When your true count is at or beyond that number, you deviate.
| Your hand | Dealer up | Basic play | Deviate to | Index |
|---|---|---|---|---|
| Insurance | A | No | Take it | +3 |
| 16 | 10 | Hit | Stand | 0 |
| 15 | 10 | Hit | Stand | +4 |
| 10,10 (20) | 5 | Stand | Split | +5 |
| 10,10 (20) | 6 | Stand | Split | +4 |
| 10 | 10 | Hit | Double | +4 |
| 12 | 3 | Hit | Stand | +2 |
| 12 | 2 | Hit | Stand | +3 |
| 11 | A | Double | Double | +1 |
| 9 | 2 | Hit | Double | +1 |
| 10 | A | Hit | Double | +4 |
| 9 | 7 | Hit | Double | +3 |
| 16 | 9 | Hit | Stand | +5 |
| 13 | 2 | Stand | Hit | -1 |
| 12 | 4 | Stand | Hit | 0 |
| 12 | 5 | Stand | Hit | -2 |
| 12 | 6 | Stand | Hit | -1 |
| 13 | 3 | Stand | Hit | -2 |
The Fab 4
These are the four most valuable late-surrender deviations, telling you to surrender certain hands at or above a given count when the game offers surrender. Confirm the exact hands and indices for your rules before publishing them, since surrender availability changes the set.
How index numbers read
A positive index means deviate when the true count is that number or higher. A negative index means deviate when the true count is that number or lower. Reading the sign correctly matters, and the trainer's deviation drill enforces it.
Priority
Do not try to learn every index at once. Insurance first, then the plays that come up most and earn the most. Diminishing returns set in quickly past the top few.