The outer fences were constructed in a number of phases, starting with the initial fortification of the border from May 1952. The first-generation fence was a crudely constructed single barbed-wire fence (Stacheldrahtzaun) which stood between 1.2 and 2.5 metres (3.9 and 8.2 ft) high and was built very close to the actual border line.[65] This was replaced in the late 1950s with parallel rows of more strongly constructed barbed-wire fences, sometimes with concertina wire placed between the fences as an additional obstacle.[66]