"A soldier would usually occupy a front line trench for only a week at a time.
"There is an important thing to understand about trench warfare and what it was like to be in the trenches. It was not the same people in the same trench all through the war. The army realized that even in quiet periods, being in the front line was a terribly wearing experience.
"At any moment, if you put your head above the parapet, a sniper might get you; at any moment, a trench mortar or shell might land among you, killing and maiming. Consequently, people there are living in a state of great anxiety, which if continued for long, would wear them down; and they would wear down pretty rapidly to the point where they can't be used again.
"To avoid this, the army was constantly recycling people, having them in the front line a week at a time, then moving them to reserve trenches, then moving them out of the lines altogether (giving them time to recuperate), and then bringing them back again."