Trench systems consisted of three lines of defence. The front-line trench was supported by a support trench and behind that a reserve trench. Each one of these trenches was connected to one another by communication trenches. The zig-zagged pattern of trenches was a defensive feature that protected against enemy fire and shell explosion.

The features of a trench:

Typically several metres deep with sand bags above head height to protect against sniper fire
The fire step was used by soldiers to peer over no-man's land and used for shelter during shelling or bad weather
At regular intervals there would be a machine-gun post
There was coils of barbed wire laid out across both sides, to slow opposition
The guns of the heavy artillery were located behind the reserve line; used to bombard enemy trenches before an attack

Typically over a 32-day period a soldier would spend:

8 days in a front-line trench
8 days in a reserve trench in case of an attack
16 days away from the front (usually in the nearest town or village)

Below is a cross-section of a trench.

British sector of the Western Front 1914-1918