With over 1000 surviving examples, the T-O diagram is the most common geographical depiction of the world in the Latin Middle Ages. It depicted the three parts of the known world in a memorable and easily replicable way. The origins of the diagram are still unclear and the subject of debate: Some scholars assert that its origin is antique - that is, rooted in non-Christian traditions - while others argue for its formation in a Christian context. The debate on the origins of the T-O diagram is directly linked to the question of whether or not it can be interpreted as a Christian sign of the world. This paper considers two questions: First, whether or not the T-O diagram was ever originally intended to be a Christian sign, and second - independently from this - since when the diagram was traceably interpreted as a religious image of the world. While a Christian design of the diagram is possible (and in the author’s view even probable), it cannot be proven beyond a doubt; however, it is clear that the diagram was understood as a Christian sign by contemporaries from the late eighth and early ninth century onwards.