The Book of Trench Warfare

It was the preferred plan on both sides that their troops sleep in fields and forests as they briskly marched forward to the terror-struck cities of their timid and surrendering foes – but other sleeping arrangements had to be made when it was decided that trenches were necessary. Officers in forward trenches would sleep in shifts within muddy little rooms called dugouts and the enlisted men would get something worse; dubbed, shelters, these holes were simply rectangular caves carved into the walls of the trench:

Click here to see a 1915 ad for British Army military camp furniture.