A trench system may begin simply as a collection of foxholes hastily dug by troops using their entrenching tools.
These holes may subsequently be deepened so that a soldier can safely stand up in one of them, and the individual foxholes may be connected by shallow crawl trenches.
They are in what was previously a German trench at Ovillers-la-Boisselle on the Somme, July 1916.
When able to rest, soldiers in front line trenches would try and shelter from the elements in dugouts.
These varied from deep underground shelters to small hollows in the side of trenches – as shown here.
This rare photograph shows the moment when the first men go over the top during a raid in spring 1917.
Trench warfare, warfare in which opposing armed forces attack, counterattack, and defend from relatively permanent systems of trenches dug into the ground.