Sign of Elo coefficient

Elo coefficient is defined through winning/losing probability relative to other players. Quite regardless of the absolute value of the coeffients involved.

It follows that Elo coefficient has no upper bound and no lower bound. Someone who defeats a 2500 rated player 76 % of time by definition is rated 2700; someone who loses to a 1500 rated player 76 % of time is by definition rated 1300.

The highest limits of natural and artificial intelligence are under heavy attention. Chess engines have been better than humans for some time. What is the highest rating currently held by a program? 3000? 3500? 4000?

But the lack of lower bound means that there is no objection to holding a negative Elo rating.

How common is measured and documented negative Elo?