The second, or main line of resistance, is called the support line. In the strong points of the centers of resistance are the reserves and the headquarters of the battalion commanders. The third line is called the covering line of the artillery, because it defends the batteries distributed behind it. It also contains the reserve troops of the sector, a battalion usually holding each strong point. Here are established the headquarters of the colonel and the different central depots of the sector. The above three lines constitute one position. There may be several positions in depth at distances of three miles or more apart. Usually there are two positions and sometimes three. It is said that the Germans have as many as six positions in depth in places. The first position only, of course, is permanently occupied. Principles of the Organization =Advantages of the Continuous Trench.= The continuous or curtain trench between support points must not be eliminated for several reasons. The trench will deceive the enemy as to the location of the elements of support points. If this trench were absent the enemy could easily locate these, and subject them to concentrated artillery fire. The position cannot be hidden from airplane observation but the essential elements, such as emplacements, dugouts, etc., must be concealed. [Illustration: Traces of Trenches Used to obtain Flanking Fire Types of Communicating Trenches.] The curtain trench serves for communicating purposes for lateral movements of troops and for liaison between neighboring units. The absence of the curtain trench would leave breaches in the line where in a general attack units could concentrate and break through. In such a general attack the curtain trench is occupied and defended by the reserves. Barbed wire, of course, protects the entire front. These trenches also serve, if necessary, as cover for reserves acting as reinforcements to the adjacent support points, as there is only sufficient shelter in a support point for its own permanent garrison. Lastly they may serve as trenches of departure in the offensive. =Importance of Flanking Fire.= It is possible to have the support points at intervals because flanking fire from these will command all ground in front. The importance of flanking fire cannot be overestimated. Everything must be done to obtain it. [Illustration: Type of a Good Entanglement in front of a Firing Line Without Salients To Obtain Flanking Fire] Sometimes it is impossible to obtain flanking fire because of lack of salients. If this is true the trace of the first line trench must be rectified to give flanking fire along the line as per diagram. When neither of these two methods is possible small combat posts with machine guns are located in advance of the line to obtain flanking fire.