Traverses
Front-line trenches were not dug in straight lines. Otherwise, if the enemy had a successive offensive, and got into your trenches, they could shoot straight along the line. The French tended to build zig-zag trenches. However, the British Army preferred a system where each trench was dug with alternate fire-bays and traverses. Whereas fire-bays were straight sections of trenches, traverses were built at angles. This limited the effect of enfilade fire or shell-burst.
Primary Sources
(1) Arthur Behrend was a balloon observer. In his memoirs An Adjutant in France, Arthur Behrend described what the front-line looked like from the air.
At six hundred feet we were free of most earthly noises, and again I looked down. For the first time I saw the front line as it really was, mile upon mile of it. Now running straight, now turning this way or that in an apparently haphazard and unnecessary curve, now straight again, it stretched roughly north and south till it vanished in both directions. The landscape was alive with the puffs of bursting shells.
(2) Robert Graves, Goodbye to All That (1929)
I went round by myself. The men of the working-party, whose job was to replace the traverses, or safety-buttresses, of the trench, looked curiously at me. They were filling sandbags with earth, piling them up bricklayer fashion, the headers and stretchers alternating, then patting them flat with spades. The sentries stood on the fire-step at the comers of the traverses, stamping their feet and blowing on their fingers. Every now and then they peered over the top for a few seconds. Two parties, each of an N.C.O. and two men, were out in the company listening-posts, connected with the front trench by a sap about fifty yards long. The German front line stretched some three hundred yards beyond. From berths hollowed in the sides of the trench and curtained with sandbags came the grunt of sleeping me.
(3) John Raws, letter to his mother (9th July 1916)
The fortification consists of breastworks, built up high to the front, with just a little shallow trench dug behind. The reason is that drainage is so difficult. These breastworks are made of millions of tightly-made sandbags laid one upon the other, packed well together. Every eight yards there is an island traverse, a great mound of earth and sandbags strengthened by rivetting, round which the trench winds. This is to localise the explosion of shells or prevent an enemy who might reach the flank being able to pour fire right down the length of a trench. There are communication trenches back every few yards and innumerable succeeding lines for the main army. The whole network extends in most places for three or four miles. The dug-outs are all in lines, but mostly along the communication trenches.
When there is no excitement there are about two sentries to every sector of say 9 yards on watch, and one officer for the company. The rest are in the dugouts. When a bombardment comes or there is a gas alarm, everyone rushes out and takes what cover one can in the front trench, awaiting developments. Against the front breastwork we have a step, about two feet high, upon which men stand to shoot. When there is a bombardment nearly everyone gets under this step, close in against the side.