Doubling from the Bar

by Mary Hickey
11 July 2017


Mary Hickey

Doubling from the bar can be scary, because almost by definition these cubes will usually be weaker than ones where you have both feet on the ground. Since you have one man you must move before you can do anything else, your choices are automatically constricted. There also tends to be a potential disaster if you stay out, and that can happen even if all your opponent has is a one point board. (Marty Storer calls this the "Boxcars Bonus".)

The most common time when doubling from the bar will be stronger than if you are on the board, is when staying out may be good for you. We'll see an example of this in Problem D. This can happen even if you are closed out, if your opponent has a man or men trapped in your home and has been forced to close you out before he can run.

A key factor in correct doubles from the bar is volatility. Your best rolls from the bar will tend to be decisive market losers unless your opponent does something special on the rebound. If they aren't, and your opponent will have plenty of remaining play after you do your worst, then caution may be the best policy.

Let's look at some of these decisions taken from real games, where sometimes the players hit the target straight on, but other times shot themselves in the foot:

Problem A.



is Player 2

score: 0
pip: 115
Unlimited Game
pip: 120
score: 0

is Player 1
XGID=---aADCCB---b---Ac-e-baa-A:0:0:1:00:0:0:0:0:10
on roll, cube action?

Analyzed in Rollout No double Double/Take
  Player Winning Chances: 63.45% (G:26.32% B:1.35%) 63.51% (G:26.85% B:1.38%)
  Opponent Winning Chances: 36.55% (G:11.66% B:0.61%) 36.49% (G:11.92% B:0.63%)
  Cubeless Equities +0.423 +0.854
Cubeful Equities
No double:+0.514 (-0.094)±0.013 (+0.501..+0.527)
Double/Take:+0.607±0.020 (+0.587..+0.628)
Double/Pass:+1.000 (+0.393)
 
Best Cube action: Double / Take
Rollout details
1296 Games rolled with Variance Reduction.
Dice Seed: 49264472
Moves: 3-ply, cube decisions: XG Roller
 
Double Decision confidence:100.0%
Take Decision confidence:100.0%
Duration: 5 minutes 33 seconds

eXtreme Gammon Version: 2.10



Both players got this double and take right over the board. It's certainly an example of volatility, since plenty of action can begin right now. That volatility requires the double, but since nothing has actually happened yet, the take is huge.

Speaking of volatility—on the four rolls where White fans, does Black have an immediate recube? XGR++ says yes by .066, enough that we can believe the recube is legitimate. But even if it were closer to the ragged border or even a bit short, it could be a good move to recube anyway as a way of telling your opponent you're not easy to push around. And if he is feeling a bit snakebit after staying out, who knows, he might pass!
 
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