Every move is compared with GNU Backgammon's best move. The grade depends on how much equity it gives up:
| Best move | The engine's first choice |
| Excellent | Less than 0.01 |
| Good | 0.01 to 0.04 |
| Inaccuracy | 0.04 to 0.08 |
| Mistake | 0.08 to 0.16 |
| Blunder | 0.16 or more |
These match GNU Backgammon's own marks: doubtful from 0.04, bad from 0.08, very bad from 0.16. Cube decisions are graded the same way.
Equity is the average number of points you'd score per game from a position, counting a gammon as 2 and allowing for the cube. A move can win slightly more often and still be worse, because it loses more gammons or wins fewer.
For example, a move that wins 4% fewer games has already lost about 0.08, because each of those games swings 2 points (a loss instead of a win). Every 1% more gammons lost, or fewer gammons won, costs another 0.01, and the cube can make the swing bigger. So a 4% drop in wins can add up to a blunder.
Losing 0.10 is like giving away 5 games in 100. The engine's numbers are estimates: differences under about 0.02 are close calls.
Tap the best move on a move card to see what it does differently: hits, how many rolls hit your blots, points made or given up, back checkers and the engine's odds. The engine itself doesn't give reasons, so these are the concrete differences between the two positions.
Error rate is the equity lost per decision, in thousandths of a point. Under 5 is world class, 5 to 8 expert, 8 to 12 advanced, 12 to 18 intermediate. It needs 10 or more decisions to mean much. Luck adds up how much better or worse each roll was than an average roll.
Expert plays GNU Backgammon's "expert" setting: it judges each move by the position it leads to, without looking ahead at the next rolls. Grading looks two rolls ahead, so it catches Expert's occasional slips. Casual and Weak add random noise to how the engine rates each option, so they sometimes pick a worse one. Over 60 to 80 simulated games each, the error rates were about 3 (world class) for Expert, 17 (intermediate) for Casual and 33 (beginner) for Weak.
One game tells you little: at the same level, single games ranged from roughly half to one and a half times those numbers.
Single games are money play with the doubling cube. The Jacoby rule applies (gammons only count once the cube has been turned) and beavers aren't allowed. Matches go to 3, 5 or 7 points with the Crawford rule, and the engine's advice takes the score into account.
GNU Backgammon is a neural-network engine. Here it searches 2 ply, its "world class" setting. It loads in a few seconds (3 MB the first time); until then a simpler built-in engine fills in.
GNU Backgammon is free software under the GPL. This app uses the browser build from github.com/hwatheod/gnubg-web.