This comprises the line of fire-trenches, with covering fire-trenches, support trenches, support dugouts and reserve dugouts (in other words, it is an exact duplication of the front line system), far enough behind the front line that in the event of the first system being taken, the second line is ready to be taken up by the troops driven out of the front line, and receive the support of troops lying in brigade or divisional reserve. The distance of the second line behind the first is roughly a mile, and while the first line is held, operations of the second often provide useful accommodation for the machine guns and artillery, both for firing purposes, observation and shelter.

The time available for siting and constructing second line system, and the freedom from all disturbances which bother the front line system, should result in its being impregnable. It is an ideal system provided those responsible for its siting and construction are thoroughly and practically experienced with first line conditions and profit by that experience and former mistakes.