'I'm 1875. What's your rating?' is a typical greeting from one chessplayer meeting another for the first time.
The other player, rated 1750, knows immediately that a game between them will be a tough battle.
A draw will be a satisfactory result and a win will be a small upset.
What is a rating and how is it calculated?
Perhaps most importantly, a rating is only meaningful relative to other ratings.
In July 1989, when Garry Kasparov became the first player to break the 2800 barrier, his accomplishment was by comparison with all other ratings, current and historical.
Vladimir Kramnik's FIDE rating chart, shown at the bottom of the page, shows the evolution of his rating after winning the Braingames World Championship from Kasparov in 2000 through his successful defense of the unified title against Veselin Topalov in 2006.
The rating system may seem mysterious, but it is grounded in statistical theory.
Obviously, two players with the same rating should have an equal chance of winning against each other.
Less obviously, the same rating difference implies the same chance of winning.
A player rated 2400 playing against a player rated 2200 has the same chance of winning as a 1400 against a 1200.
The rating difference is 200 points in both cases.
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