Arpad Elo was a physics professor and a chess master. In the 1960s, he designed a rating system for the United States Chess Federation. It was later adopted by FIDE, the international chess body. The Elo system calculates a player's skill based on wins and losses against opponents. Beat a stronger player, gain more points. Lose to a weaker one, lose more. The math is simple. The impact is huge.

ELO has spread beyond chess. Video games use it for matchmaking. League of Legends, Counter-Strike, and chess.com all use Elo-based systems. The goal is fair matches. A player with a 1500 rating should face another 1500. The system predicts the probability of a win. It is not perfect. It can be gamed. But it is the standard.

Key ideas

• Rating: a number representing skill.
• K-factor: how much a single game changes your rating.
• Expected score: the probability of winning based on ratings.
• Rating inflation: when the average rating creeps up over time.

Elo is not just for games. It is used in sports, debate, and even dating apps. Anywhere you need to rank people by skill, Elo can help. It is a simple idea with a lasting legacy. Arpad Elo died in 1992. His rating system outlived him.