The line was long and built for a garrison of twenty divisions, one every . Telephone cables were deeply buried and light railways built to carry supplies to the defences. The position had two trenches about apart, with sentry garrisons to occupy the front trench. The main line of defence was the second line, which was equipped with dugouts for most of the front garrison. Fields of barbed wire up to deep, were fixed with screw pickets in three belts wide and apart, in a zig-zag so that machine-guns could sweep the sides placed in front of the trench system. Artillery observation posts and machine-gun nests were built in front of and behind the trench lines. Where the lay of the land gave observation from behind the system, it was built on reverse slopes (a ), with a short field of fire for the infantry, according to the experience of the Western Front defensive battles of 1915 and 1916, when forward-slope positions had been smashed by observed Franco-British artillery-fire.