Harkness was responsible for introducing Swiss system tournaments to the United States, and also introduced the Harkness rating system, which was a precursor to the Elo rating system. One method of tiebreaks in Swiss systems, where players tied on points are ranked by the sum of the opponents' scores minus the top score and the bottom score, is named after him. For his services, Harkness is in the U.S. Chess Hall of Fame.
Harkness devised a rating system that was used by the US Chess Federation from 1950 to 1960. When a player competes in a tournament, the average rating of his competition is calculated. If the player scores 50% he receives the average competition rating as his performance rating. If he scores more than 50% his new rating is the competition average plus 10 points for each percentage point above 50. If he scores less than 50% his new rating is the competition average minus 10 points for each percentage point below 50 (Harkness 1967:185–88).