Given that Elo ratings appear to only have been around officially since 1970, how do you compare top chess players that were not scored using Elo (e.g. their careers came before ratings existed)?
Jeff Sonas at chessmetrics calculated ratings for players going way back, using the games we have available. Gives a pretty good picture of players relative strength.
Note this is up to around 2005, so you won't see players like Carlsen on there.