Forum Archive : Terminology


From:   Michael J. Zehr
Address:   tada@athena.mit.edu
Date:   1 March 1996
Subject:   Backgammon terms (was Re: JellyFish 2.0 Release)
Forum:   rec.games.backgammon
Google:   4h79i2$io0@senator-bedfellow.MIT.EDU

Dave Biggs writes:
> Fredrik Dahl says:
> > In the Tutor you can have positions evaluated with equity and
> > the 6 probabilities.
> Dumb newbie type questions...what do you mean by equity & 6
> probabilities? I've seen both mentioned in the periodical "Inside
> Backgammon".
> And what do you mean by "rollouts"?

Equity is used in several slightly different ways all generally meaning
the value of a position to a particular side.  Here's an example of the
most common meaning:

Suppose I have two checkers on my two point, you have two checkers on
your ace point, and I'm on roll.  If I roll any 1 except 11, I bear my
two checkers off and win the value of the cube.  Otherwise, you roll and
bear two checkers off and I lose.  I win 26/36 times and lose 10/36
times.  What I expect to win is (26-10)/36 or 16/36.  Thus my equity is
16/36 or .444.

Suppose I am considering doubling.  If I double and you take, I win just
as many games but for twice the value, so my equity becomes .889.  Since
I increase my equity by doubling, I want to double.

Should you take or not?  If you drop, you lose the game all the time and
your equity is -1.00.  If you take, your equity is -.889.  Since this is
an improvement, you take.

Some important observations:

Equity depends on the cube position.  If the cube is in the center or I
own it, my equity in this position is .889, otherwise it's .444.  In
more complicated positions, my equity by doubling is less than twice the
equity by not doubling because the opponent gets some value out of being
able to redouble.

Opponents have equal and opposite equity.  (This must be true because
anything one side loses, the other side wins.)

In this sense of the word, equity is being used to describe the value of
a position assuming perfect play by both sides.

Rollouts are used when a position is more complicated than being able to
simply list the winning rolls for one side.  A rollout is playing the
game to completion over and over and over and keeping track of the
number of points won or lost by a side.  The existence of computer
programs that are good enough to do a reliable rollout has greatly
increased our understanding of the game in recent years.  Rollouts by
hand are a great way to understand a position, but are necessarily much
slower.  For a serious backgammon student, the rollout featuer of
JellyFish is an important study aid.

"6 probabilities" -- I believe this refers to the chance of winning a
single game, a gammon, or a backgammon, and the chance of losing the
same.  This gives greater detail than simply the equity of a position.

For example, you can have an equity of .50 by winning a gammon .5 times
and losing a single game the rest of the time (.5*2 - .5 = .5) or by
winning a single game .75 times and losing a single game .25 times (.75
- .25 = .5).  When looking at a position for a match, gammons and
backgammons are not always worth twice and thrice a normal game.  (For
example, at double match point, one of the two positions mentioned above
is much better!)

Other uses of equity:

Sometimes it's used to compare the relative strengths of players rather
than a specific position assuming perfect play by both sides.  For
example, I might say that my equity against some other opponent is +.1,
i.e. I expect to win .1 points per game I play that person.  (This
represents a fairly wide gap in skills, and I would be very happy to
play this person for money of course! <grin>)  On the other hand, my
equity against Kit Woolsey might be -.05 pts per game.

"Equity surge" -- You can say this when you've just gotten a great roll.
Your equity suddenly went up.

I hope these explanations helped.  You can also look for the Backgammon
FAQ (posted here periodically or available through anoymous ftp at

-michael j zehr
Did you find the information in this article useful?          

Do you have any comments you'd like to add?     



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"Dance"  (William R. Tallmadge, May 1998) 
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"Equity"  (Gregg Cattanach, Aug 2000) 
"Equity"  (Gary Wong, Dec 1998) 
"Equity"  (Chuck Bower, Oct 1996) 
"Equity"  (Michael J. Zehr, Mar 1996) 
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"Freeroll"  (montygram, Nov 2005) 
"Gammon price"  (Ron Karr, Aug 1996) 
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"Gammon-go" (GG) and "gammon-save" (GS)  (Marty Storer, Oct 2002)  [GammOnLine forum]
"Gammon-go" (GG)   (Chuck Bower, Jan 2004) 
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"Holding game"  (Alan Webb+, Dec 1998) 
"In the box"  (Ken Bame+, Sept 2012) 
International phrase dictionary  (David Allen Sorensen, Sept 1997) 
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"Kamikaze play"  (Bill Patterson+, June 2011) 
"Kauder paradox"  (Carl Tait+, Nov 1995) 
"Latto paradox"  (Jean-Pierre Seiman+, July 2004) 
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"MCV"  (Mislav Radica+, Oct 2009) 
"PRaT"  (Raccoon+, Jan 2007)  [GammOnLine forum]
"Phantom double hit"  (Marty Storer, May 2010) 
"Polish prime"  (Jason Lee+, Jan 2006)  [GammOnLine forum]
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"Pure play"  (Casey Forrest+, Feb 1996) 
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"Short play"  (AJ+, July 2012) 
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"Volatility"  (Michael J. Zehr, June 1998) 
"Wash"  (Hardy Hübener+, Sept 2004) 
"Wash"  (Brian Sheppard, July 1997) 
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