/jedo/ → /edo/ Compound of 江 (e, “cove, inlet”) + 戸 (to, “entrance, gate, door”), likely in reference to the local geography. The to changes to do as an instance of rendaku (連濁). First mentioned in the 吾妻鏡 (Azuma Kagami, “Eastern Mirror”, an historical chronicle), started in 1180 and completed after 1266, in a section dated to 1180.