The basic structure of the border that divided Berlin developed in the course of the 1960s. To the west, a wall of concrete slabs was erected and capped by a wide cylindrical pipe to prevent an easy grip. Behind it lay a strip of land approximately fifty meters wide studded with barricades, detectors, watch towers, and guard dogs on long leads. This part of the frontier barriers soon became known as the "death strip'' because the East German border guards used their weapons to thwart attempts at escape. To the east, too, the border was sealed by a wall. Unlike the population on the western side, however, the population of the GDR was given little opportunity to see the frontier for themselves. Usually designated as prohibited military zones, the areas adjoining the border itself were off limits. Even the GDR citizens who lived in these districts were required to have special permits or special identification in order to pass. This also went for their closest relatives living in East Berlin or in the GDR who simply wanted to pay a visit. The "pass" had to be applied for long in advance and was not always granted.