Beatus map in the context of "Beatus of Liébana"

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.

The Commentary on the Apocalypse (Commentaria in Apocalypsin) is a Latin commentary on the biblical Book of Revelation written around 776 by the Spanish monk and theologian Beatus of Liébana (c. 730–after 785). The surviving texts differ somewhat, and the work is mainly famous for the spectacular illustrations in a group of illustrated manuscripts, mostly produced on the Iberian Peninsula over the following five centuries. There are 29 surviving illustrated manuscripts (many incomplete or fragments) dating from the 9th to the 13th centuries, as well as other unillustrated and later manuscripts. Significant copies include the Morgan, Saint-Sever, Gerona, Osma, Madrid (Vitr 14-1), and Tábara Beatus codices.

Most unusually for a theological work, the imagery seems to have been included from the start, and is considered to be the work of Beatus himself, although the earliest surviving manuscripts date from about a century after he wrote the book. After about another century, around 950, the size and number of illustrations was expanded. Manuscripts of the work are typically referred to just as a Beatus. They included a Beatus map, a version of the medieval type of world map called the T and O map with added details; this is supposed to have been created by Beatus. It has only survived in some copies.