Support Trenches. The second or support line is established at such a distance that the first line is covered by its fire if the latter is captured.
Also the second line should be placed so that it will be out of the zone of dispersion if the first line is the artillery target either of your own or of hostile fire. This places the distance between the two lines at from 150 to 400 yards. At this distance also rifle grenade fire can be used in conjunction with an artillery preparation for a counterattack against the first line.
The organization of the second line is the same as that of the first; in other words, the same principles and elements of defense are used: making employment of barbed wire, flanking fire, cover trenches, etc.
The second line is the main line of resistance and must be made as nearly impregnable as possible. For this purpose it must be thoroughly and completely organized.
In the support line are the reserves of the C. of R. and for their proper shelter they should have very deep dugouts, and all the requisites for the comfort and rest of the troops.
Redoubts. Behind the support line is a redoubt containing the command post of the battalion commander. It is the last strong point of the center of resistance and is organized for an all-around defense.
The line of redoubts is the last line of serious resistance. If the enemy penetrates this line a breach has been made in the position at this point.
Boyaux and Communicating Ditches. As we have said before trenches for communicating purposes must be dug both in depth and laterally. We shall call those from front to rear boyaux, and those running laterally communicating ditches.
The number of boyaux is not fixed as that of the different lines. There must be at least one between each support point and its center of resistance.
Usually boyaux are used for movements in both directions. Often certain boyaux are designated for entrance only and others for evacuation. The numbers of the latter are usually less than the former.