Editing Trench warfare (section)

=== Layout ===
{{Unreferenced section|date=November 2024}}
Trenches were never straight but were dug in a [[zigzag]]ging or stepped pattern, with all straight sections generally kept less than ten yards. Later, this evolved to have the combat trenches broken into distinct [[wikt:fire bay|fire bay]]s connected by traverses. While this isolated the view of friendly soldiers along their own trench, this ensured the entire trench could not be [[Enfilade and defilade|enfiladed]] if the enemy gained access at any one point; or if a bomb, grenade, or shell landed in the trench, the blast could not travel far.

[[File:Aerial view Loos-Hulluch trench system July 1917.jpg|thumb|left|Aerial view of opposing trench lines between Loos and Hulluch, July 1917. German trenches at the right and bottom, British at the top-left.]]
Very early in the war, British defensive doctrine suggested a main trench system of three parallel lines, interconnected by communications trenches. The point at which a communications trench intersected the front trench was of critical importance, and it was usually heavily fortified. The front trench was lightly garrisoned and typically occupied in force only during "stand to" at dawn and dusk. Between {{convert|70|and|100|yd|m|abbr=on|order=flip|round=5}} behind the front trench was located the support (or "travel") trench, to which the garrison would retreat when the front trench was bombarded.

Between {{convert|100|and|300|yd|m|order=flip|-1}} further to the rear was located the third reserve trench, where the reserve troops could amass for a counter-attack if the front trenches were captured. This defensive layout was soon rendered obsolete as the power of artillery grew; however, in certain sectors of the front, the support trench was maintained as a decoy to attract the enemy bombardment away from the front and reserve lines. Fires were lit in the support line to make it appear inhabited and any damage done immediately repaired.

Temporary trenches were also built. When a major attack was planned, assembly trenches would be dug near the front trench. These were used to provide a sheltered place for the waves of attacking troops who would follow the first waves leaving from the front trench. "Saps" were temporary, unmanned, often dead-end utility trenches dug out into no-man's land. They fulfilled a variety of purposes, such as connecting the front trench to a listening post close to the enemy wire or providing an advance "jumping-off" line for a surprise attack. When one side's front line bulged towards the opposition, a [[Salient (military)|salient]] was formed. The concave trench line facing the salient was called a "re-entrant." Large salients were perilous for their occupants because they could be assailed from three sides.

Behind the front system of trenches there were usually at least two more partially prepared trench systems, kilometres to the rear, ready to be occupied in the event of a retreat. The Germans often prepared multiple redundant trench systems; in 1916 their [[Battle of the Somme|Somme]] front featured two complete trench systems, one kilometre apart, with a third partially completed system a further kilometre behind. This duplication made a decisive breakthrough virtually impossible. In the event that a section of the first trench system was captured, a "switch" trench would be dug to connect the second trench system to the still-held section of the first.