Hereford Mappa Mundi in the context of "Hereford Cathedral"

A T and O map or O–T or T–O map (orbis terrarum, orb or circle of the lands; with the letter T inside an O), also known as an Isidoran map, is a type of early world map that represents the Afro-Eurasian landmass as a circle (= O) divided into three parts by a T-shaped combination of the Mediterranean sea, the river Tanais (Don) and the Nile. The origins of this diagram are contested, with some scholars hypothesizing an origin in Roman or late antiquity, while others consider it to have originated in 7th or early-8th century Spain.

The earliest surviving example of a T-O map is found in a late-7th or early-8th century copy of Isidore of Seville's (c. 560–636) De natura rerum, which alongside his Etymologiae (c. 625) are two of the most common texts to be accompanied by such a diagram in the Middle Ages. A later manuscript added the names of Noah's sons (Sem, Iafeth and Cham) for each of the three continents (see Biblical terminology for race). A later variation with more detail is the Beatus map drawn by Beatus of Liébana, an 8th-century Spanish monk, in the prologue to his Commentary on the Apocalypse.

A mappa mundi (Latin [ˈmappa ˈmʊndiː]; plural = mappae mundi; French: mappemonde; Middle English: mappemond) is any medieval European map of the world. Such maps range in size and complexity from simple schematic maps 25 millimetres (1 inch) or less across to elaborate wall maps, the largest of which to survive to modern times, the Ebstorf map, was around 3.5 m (11 ft 6 in) in diameter. The term derives from the Medieval Latin words mappa (cloth or chart) and mundus (world).

Around 1,100 mappae mundi are known to have survived from the Middle Ages. Of these, some 900 are found illustrating manuscript books and the remainder exist as stand-alone documents.