How to think the idea of the border in a way that takes its changing material reality fully into account? A starting point might be the three fundamental elements of transmission proposed by mediology: the technology that transmits the message; the message itself and its symbolic meaning; and the institution that not only enables its communication across space but ensures its transmission across time.footnote1 In material terms, the border is first of all ‘natural’: rivers, streams, flat stones, elms and oaks allow us to identify a boundary. It may be man-made: a succession of landmarks, crosses, plaques—but also walls, trenches or barbed wire. It may be discontinuous: a string of forts, military encampments or customs posts.

These physical markers on the ground would mean nothing without the words that lend them meaning and transmit their memory. Fortresses and customs posts cannot function without the military hierarchy or excise administration to which they belong. At first, oral tradition conveyed the remembered meaning of the boundary marker, the lone tree. More often, written forms—treaties or charters—or images, such as the map, present the message: a given territory stops here. But maps and charters have no bearing without institutions to preserve and under-write what their information is worth. So: no frontier without a chancellery and an official records office; and no maps without surveyors employed by those in power. In the age of democracy, when it became necessary to take public opinion into account, maps and atlases helped citizens to learn—under the teacher’s rod, if necessary—the outlines of the state that they could one day be called upon to defend.

Each epoch, then, can be distinguished by the institutions of its frontiers, the material vectors of their inscription on the land and the forms of their transmission. To test this hypothesis, we’ll look at three different instances, each magnified to identify its logic. First, the medieval border which, from around 800 ad, separated the Germanic tribes from their Slavic neighbours; next, some of Europe’s feudal boundaries; finally, the emergence of modern state frontiers in the era of print and cartography. For each age, the wager is that there is a political form and a usage of the frontier which corresponds to its technological stage.

First, a hypothesis: we have been speaking of a precisely drawn, linear border, not a vague, undifferentiated space. This, however, is by no means self-evident. Ancient historiography insisted on the fluid character of borders beyond the Roman Empire. Modern historians, by contrast, argue for precise lines of division. This underlines the tension between an imperial conception of the frontier—well-marked, punctuated by the presence of forts and soldiers, capable of controlling entrance to the empire, delineating inside from outside—and the practice of the same border as a place for commercial transaction, contraband, immigration and displacement of peoples.

First-century ad descriptions of the spaces east of the Rhine insisted on the non-linear character of the borders between the peoples, of whom the writers knew only by hearsay. The political imagination of the times saw, in the forests inhabited by the Germanic tribes, alternations between full and empty zones. Thus Caesar in the Gallic Wars emphasized the uninhabited spaces which, he said, separated the Germanic peoples from each other:

They think that the greatest glory of a nation is to have the largest possible wilderness beyond its borders, as that signifies the number of cities that could not withstand the glory of its arms. It is said that on one side of the Suebian border there is emptiness for 600,000 paces. On the other side, their neighbours are the Ubii [Franks], who form a significant nation.

Caesar partially contradicts himself, however, when he talks of the importance of commerce in this region of the Rhine—and when he goes on to describe the misfortunes of the Menapians, a Belgic tribe installed on both sides of the Rhine yet vulnerable to attack, since they lacked a line of defence.