The characters of Kamen Rider Decade exist amongst various iterations of reality within the series referred to as an A.R. World (Another Rider's/Alternate Reality World). The Nine Worlds (9つの世界, Kokonotsu no Sekai) are the A.R. Worlds that are based on the previous entries of the Kamen Rider Series that have aired during the Heisei period of Japanese history. Each differs in some way from the series on which it was based.