Elo refers to a player's skill rating, as told by the Elo Rating system developed by Arpad Elo for chess.
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Every player starts with 1200 points in the Elo Rating System. In Stick Empires, the ratings for Deathmatch and Regular games are seperate from each other.
When a player wins a game, they take points from the loser. The amount of points taken is dependant on the diffirence in their ratings. If a low ranked player beat a high ranked player, the low ranked player would gain many more points than if the high ranked player beat the low ranked player.
Elo is meant to represent a player's skill and probability of winning. If two players have the exact same Elo rating, they each have a 50% chance of winning. If one player has exactly 100 more points than the other, then the higher-ranked player has approximately 64% chance of winning. If the difference was 200 points, the higher ranked player has a 76% chance of winning, and so on.
Beginners start at the elo of 1200. Elo gains and losses are based on the difference in elo between the two players, with the higher elo player gaining less for beating lower elo players and players gaining more elo for winning against players with more elo than them. The elo gains for extremely high rated players against low rated ones may be less than 0.5 elo, which can result in games seeming to not increase your elo, because it's increased by less than one.