Originally posted by Cleojuly
Which ratings are used to calculate the new ratings? The ratings they had at the beginning of the game or the ratings they had at the conclusion of the game?
The ratings used as a base of the calculation are the ratings at the moment when the game ends. Only that.
(If one or both are provisional players (not fulfilled 20 rated games) the calculation is somewhat different compared if you're a non-provisional.)
And that's correct. You can easily mis-use the calculation to blow up your rating to astronomical levels if you are cheat minded.