The characters of Kamen Rider Decade exist amongst various different 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.
The World of Kuuga(クウガの世界,Kūga no Sekai) is the version of reality where Kamen Rider Kuuga takes place. It was the world where Yusuke Onodera originated from before he joined Tsukasa in his journey. The backdrop used to arrive in this world shows a city with police cars and Mount Hitoki (灯溶山,Hitoki-yama?) in the background. Within this A.R. World, Tsukasa assumes the guise of a police officer with the talent of understanding and speaking the Gurongi language.
The World of Kiva(キバの世界,Kiba no Sekai) is the version of reality where Kamen Rider Kiva takes place. Unlike the original series, this world allows Fangires and humans to coexist, with the King's servants destroying the rebellious Fangires. The backdrop that allows entry into the World of Kiva depicts a nighttime cityscape with Castle Doran emerging from a building. Within this A.R. World, Tsukasa assumes the identity of a violinist with the talent of playing violin solo similar to those of Otoya and Wataru Kurenai.
The World of Ryuki(龍騎の世界,Ryūki no Sekai) is the version of reality where Kamen Rider Ryuki takes place. In this world, there exists the Kamen Rider Trial (仮面ライダー裁判,Kamen Raidā Saiban), a battle where the Kamen Riders fight each other within a parallel dimension known as the Mirror World (ミラーワールド,Mirā Wārudo) alongside their Contract Monsters (契約モンスター,Keiyaku Monsutā) in order for the victor to decide the fate of a defendant in a criminal case. Though no harm occurs to the Rider while in the Mirror World, a battle in the real world can be fatal and use of Vent cards outside a battle is forbidden. The backdrop used to arrive in this world shows Dragredder flying within the Mirror World as shown in a mirrored time display screen on a skyscraper. Within this A.R. World, Tsukasa assumes the identity of a lawyer and the ability to enter the Mirror World without an Advent Deck.
The World of Blade(ブレイドの世界,Bureido no Sekai) is the version of reality where Kamen Rider Bladetakes place. In this world, the Riders are high rank employees of the BOARD Corporation (株式会社BOARD,Kabushikigaisha Bōdo), and are effectively "Kamen Rider Salarymen" (仮面ライダーサラリーマン,Kamen Raidā Sararīman), whose job it is to combat the Undead once receiving clearance to use Rouzers. In BOARD, there is a ranking system with Ace (エース,Ēsu) at the top followed by King, Queen, Jack, and the numbers with Two at the bottom. The backdrop used to arrive in this world shows a hand of facedown Rouse cards with the Ace of Spades (Blade's Change Beetle) face up. Within this A.R. World, Tsukasa assumes the role of chief waiter within BOARD and is given the ability to completely destroy Undead without sealing them.
The World of Faiz(ファイズの世界,Faizu no Sekai) is the version of reality where Kamen Rider Faiz takes place. In this world, students of the Smart Brain High School (スマートブレイン･ハイスクール,Smāto Burēn Hai Sukūru?) are being attacked nightly by monsters known as the Orphnoch. The backdrop used to enter the World of Faiz is a splitscreen featuring the Autovajin in Battle Mode and blue butterflies, referencing Smart Lady, a character from the original series who was Smart Brain Corporation's mascot of sorts. Within this A.R. World, Tsukasa assumes the role of a Smart Brain High student and is able to surpass the talent intelligence of all of Lucky Clover, such as skills in tennis. Daiki's treasure in this world is the Faiz Belt, but eventually he relents and takes the unused Orga Belt instead.
The World of Agito(アギトの世界,Agito no Sekai) is the version of reality where Kamen Rider Agito takes place. Similar to the World of Kuuga, the Riders and the police battle the Gurongi. However, a new evil surfaces onto the world, monsters that they refer to as the Unknown. The backdrop used to enter the World of Agito is the elaborate tapestry seen in the opening sequence of Agito that depicts the story of Agito and the Unknown. This story arc further expands on the war between the Unknown and the Gurongi, which is hinted in both their respective series and explained more outside them. Within this A.R. World, Tsukasa assumes the role of a mailman with Toko's letter on his person. Due to the similarities to the World of Kuuga, Tsukasa more or less possesses the same talents in this world, as well as knowledge of the Unknown and the Agito Seed. Daiki's treasure in this world is the G4 Chip, but it eventually destroyed by Toko.
The World of Den-O (電王の世界,Den'ō no Sekai) is the version of reality in which Kamen Rider Den-Otakes place. Unlike the other Riders Worlds, the World of Den-O is almost identical to the setting of the original series. The only difference is the interior of the DenLiner having changed to that of the New DenLiner. The backdrop used to enter this world depicts the DenLiner moving across the sands of time, which Ryutaros doodles on later in the episode arc. Within this A.R. World, Tsukasa finds himself wearing a trenchcoat and bucket hat, the same clothing as the Past Man and given abilities similar to those of a Singularity Point. Daiki's treasure in this world is the DenLiner. The World of Den-O arc coincides with theatrical release of Cho Kamen Rider Den-O & Decade Neo Generations: The Onigashima Warship, which takes place within the continuity of Decade like Kamen Rider Den-O: I'm Born! did with Den-O. And later Daiki returns to the World of Den-O to settle things with Reiji Kurosaki who transforms into Kamen Rider G Den-O in Episode Yellow: Treasure de End Pirates, part of Kamen Rider × Kamen Rider × Kamen Rider The Movie: Cho-Den-O Trilogy. His treasures are the pistol heirloom from the Kurosaki family and the Diend K-Touch which transforms him into a Diend's Complete Form.
The World of Kabuto(カブトの世界,Kabuto no Sekai) is the version of reality where Kamen Rider Kabuto takes place. The backdrop used to enter this world depicts Tokyo Tower with an arm outstretched in homage to Tendou Souji's "walking the path to heaven" pose. Within this A.R. World, Tsukasa assumes the role of a ZECTrooper and given the ability to see in Clock Up space. Daiki's treasure in this world is the Clock Up System.
The World of Hibiki(響鬼の世界,Hibiki no Sekai) is the version of reality where Kamen Rider Hibiki takes place. In this world, humans train to become Oni within one of three rival schools of sound-based martial arts called Ongekidō (音撃道,Ongekidō, "sound-beat path"), all based on different interpretations of the writings of the Grand Master (大師匠,Daishishō), the creator of Ongekidō. The masters and students of the Ongekidō styles wear traditional Japanese clothing, Tsukasa donning such attire while in the A.R. World and given the ability to use the Ongekibou. Daiki's treasures in this world are the three scrolls containing the secrets of the three schools of Ongekidō. The backdrop used to enter this world depicts a forest with a taiko drum in the foreground. Nearly all of the original cast of Hibiki returns to reprise their roles, with only Shigeki Hosokawa (the original Hibiki) and Rakuto Tochihara (the original Asumu) absent. The episodes also show calligraphic representations of the Kamen Riders' names: "Tsukasa Kadoya" (門矢士,Kadoya Tsukasa), "Photo" (写,Utsushi), "passing through" (通,tooru), "Decade" (ディケイド,Dikeido), "Sea Cucumber" (海鼠,Namako), and Pink(桃色,Momoiro) for Decade; "Kuuga" (空我,Kūga) for Kuuga; "Peach" (桃,Momo) for Den-O; and "Thief" (盗,Tō) and "Diend" (ディエンド,Diendo) for Diend.