Shikoku , Japanese pronunciation: [ɕikokɯ] is the smallest of the four main islands of Japan. It is 300 km or 186.4 mi long and between 50 and 150 km or 31.1 and 93.2 mi at its widest. It has a population of 3.8 million, the least populated out of Japan's four main islands. It is south of Honshu and northeast of Kyushu. Shikoku's ancient names include Iyo-no-futana-shima (伊予之二名島), Iyo-shima (伊予島), and Futana-shima (二名島), and its current name refers to the four former provinces that make up the island: Awa, Tosa, Sanuki, and Iyo.

Iyo Province was a province of Japan in the area of northwestern Shikoku. Iyo bordered on Sanuki Province to the northeast, Awa to the east, and Tosa to the south. Its abbreviated form name was Yoshū (予州). In terms of the Gokishichidō system, Iyo was one of the provinces of the Nankaidō circuit. Under the Engishiki classification system, Iyo was ranked as one of the "upper countries" (上国) in terms of importance, and one of the "far countries" (遠国) in terms of distance from the capital. The provincial capital was located in what is now the city of Imabari, but its exact location is still unknown. The ichinomiya of the province is the Ōyamazumi Shrine located on the island of Ōmishima in what is now part of Imabari. The people spoke Iyo dialect.