How to play

Watch one game from a real, analysed backgammon match and guess each player's PR. 5 rounds, up to 10,000 points.

A round

1. Watch the game. Step through it move by move or let it play. You see the match length, the score, the cube, the dice and every move. You do not see the players' names, their ratings or any analysis.
2. Guess both players. Drag each slider to your guess of that player's PR. Both start empty, so no default nudges your guess.
3. Lock in. The actual PRs appear with your points, and the move list lights up: ?! an inaccuracy, ? a mistake, ?? a blunder. Jump to the worst error, or open the game in HedgeHog for a full analysis.

After 5 rounds you get your total, a round-by-round table and a grid of hits to share. There is no time limit and no streak bonus: take your time and watch the game.

What you are guessing

The answer is each player's PR over the whole match, the number OpenGammon shows for it. The game you watch is one game of that match, the one with the most decisions for both players. After the reveal you also see the game PR, how well each player did in that game alone. One game is a small sample, so the two can differ a lot.

New to PR? What is a good PR in backgammon? explains the scale.

Scoring

Each guess is scored on its relative error, e:

e      = | ln( (guess + 2) / (actual + 2) ) |
points = round( 1,000 × exp( −e / 0.35 ) )

• Each guess scores up to 1,000 points: 2,000 per round, 10,000 per run.
• Bullseye: with e under 0.05 (within about 5%) the guess scores a flat 1,000.
• Hit (green): e under 0.18, about 20% off. Near (yellow): e under 0.34, about 40% off. Both are measured on PR + 2.

FAQ

What is PR in backgammon?

PR (Performance Rating) measures how much equity a player gives away per decision compared with the best play a strong bot finds: the average equity lost per non-trivial decision, times 500. Lower is better, and 0 would be perfect play. On OpenGammon's scale a PR under 4.75 is Grandmaster level and anything above 18 is Beginner.

Is it the same as the PR in eXtreme Gammon (XG)?

Close, but not identical. OpenGammon analyses games with GNU Backgammon and counts decisions with XG's rules, so the numbers are comparable with XG's but not the same.

Why is the answer the match PR and not the PR of the game I watched?

The match PR is the number players know from their own results, and one game is only a sample of it. After the reveal you see both, which shows how far a single game can stray from the match.

Where do the games come from?

From rated matches between two people on OpenGammon, analysed by GNU Backgammon. New games are added every day. Player names, ratings and match ids never appear on this site.

Why is a small miss at a low PR worth fewer points than at a high PR?

Scoring is on relative error. The gap between a 4 and a 7 is much bigger in skill than the gap between 22 and 25, so it costs more points. The +2 in the formula keeps the very lowest PRs forgiving: guessing 3.9 when the answer is 3 is 30% off, and still a hit.

Can I see the full analysis of a game?

Yes, after the reveal. The move list shows every inaccuracy, mistake and blunder, and "Analyse with HedgeHog" opens the game in HedgeHog. The game travels inside the link, and the players are named White and Black.

Can I challenge a friend?

Share your result at the end of a run. The link shows your score, not the answers, and lets your friend play the same five games.

Does the site use cookies?

No. Your best score and a run in progress stay in your own browser. The privacy page has the details.