A support trench is usually within 30 to 300 yards of a fire trench, and may serve the purposes of covering fire trenches by skilled riflemen or an indirect machine gun fire, but their main purpose is to shelter troops from observation and shell fire, and thus their main characteristics become the size and strength of ample dugouts. Troops in the support dugouts are at hand for three purposes: Firstly, replacing of casualties occurring in the fire trenches during normal times or a hostile attack. Secondly, holding the support trenches in case the fire trenches are taken by the enemy. Thirdly, in the event of an attack on the enemy’s trenches, leading the attack by moving forward over the heads of the occupants of the fire trench, or if the latter are leading the attack, to occupy at once the fire trench when vacated. For this reason it is of vast importance that there are accessible and commodious support dugouts and communications between the immediate support trenches and front line. If this is so there will be less chance of disasters to supports and reserves coming up to make good a successful attack. There must be support dugouts even in the event of there being no support trenches, and this is very often possible owing to the lay of the land. The strength and size of these dugouts entirely depend on tactical considerations and local conditions, which are generally decided by the staff. SUPPORT POINTS These forts or strong points, as they are sometimes called, usually round or square, but which may be any shape best suited to the condition of the country in which they are placed, are generally from 100 to 300 yards behind the fire-trenches and supplementary to the support trenches. Each of these strong points contains a permanent garrison of firing troops, strongly protected with barbed wiring and sandbag revetments, and well supplied with ammunition, food and water, to enable them to withstand heavy attacks. During an attack they are used to give overhead covering fire, and for the control of ammunition and other supplies to be sent on to the firing line. It is a general rule that if the enemy take a fire line the garrison of these supporting points must hold out and remain a thorn in the enemy’s side until the last man of the garrison is killed. RESERVE DUGOUTS These dugouts protect the local reserves from which supports are supplied and are used for purposes similar to those for which the immediate support dugouts are employed, but on a great deal larger scale. These dugouts are generally near battalion headquarters and from 500 to 1,500 yards behind the firing line. The chief considerations in siting the positions of these dugouts are three: First: facilities for rapid and easy transit to the support and fire-trenches; second: concealment; third: comfort. Comfort should be secondary to the other conditions affecting the siting of the system. SECOND LINE