Historical Rules · 1 February 2013
Rule 704.5m
Official source text from the 1 February 2013 Rules release
If two or more permanents have the supertype world, all except the one that has been a permanent with the world supertype on the battlefield for the shortest amount of time are put into their owners' graveyards. In the event of a tie for the shortest amount of time, all are put into their owners' graveyards. This is called the "world rule."