They were divided into three waves. The first, formed of twelve "Rovers" or wire-crushers, moved forward at zero, 150 yards in advance of the second, to crush the wire protecting the enemy front and support trenches, and then to engage machine-guns and any special posts outside the main trenches. The second wave, formed of thirty-six "Fighting" tanks, dealt with the trenches up to and including the Blue line. The third, composed of all the remaining fighting tanks, was detailed to form up an hour and a half after zero just south of the Grand Ravine, and to proceed with the survivors of the first two to attack the Flesquières Ridge. The distribution was on the basis of one section of three tanks to a platoon frontage (_i.e._, about 150 yards). The general principle on which sections were to work was for the two outside tanks to cross a trench, turn alongside it and help to clear it, while the centre tanks carried on to the next trench, there to be joined by the two others as soon as the infantry had reached the first trench. However, in tackling a system which contained many crater posts and sap-heads, and which was thickly interlaced with short communication trenches and backed by many subsidiary trenches and detached posts, some modifications of the general principles were necessary. Special tasks were therefore given to many of the second wave sections of tanks, in which each tank was given some sap-head, crater post, or communication trench to deal with in addition to assailing the main trenches. Similarly, detailed instructions were given as to routes and individual objectives in the village fighting that was anticipated. Each tank carried on its back a huge fascine or faggot, resting on a giant pair of arms. A contrivance existed by which these arms raised the fascines off the back of the tank, and dropped them into any unusually wide trenches encountered, so as to form a stepping-stone, which enabled the tanks to keep their noses from dropping into the bottom of the trench. In spite of this precaution the Hindenburg Line was in some parts so broad and deep that a number of tanks were ditched in spite of the fascines. The first wave of the infantry followed the tanks at a distance of 150-200 yards, their orders being to assault immediately the tanks reached and opened fire on a trench. On reaching their objectives, the infantry marked with red strips of cloth the gaps in the wire, and filled in portions of the trenches to make crossing-places for subsequent tanks, cavalry, and artillery. Each tank carried for the use of the infantry Lewis gun drums, rifle ammunition, bombs, and rifle grenades. The field artillery supporting the attacks was divided into two groups, each composed of two field artillery brigades and one R.H.A. brigade. Each brigade was supported by one of these groups, that containing 256th Brigade, R.F.A., being on the right and that containing 255th Brigade, R.F.A., on the left.