2d 20D20s are used for resolving skill tests, and for rolling on certain large tables. Often, two or more d20s will be required. This is noted as Xd20, where X is the number of dice to be rolled.A dital PDF of Fallout: The Roleplaying Game - 2d20 Rules Booklet, a book which helps Overseers use the 2d20 ruleset from Modiphius