The Y shape allows plenty of air into the fire but the trench stops it being blown away.
From the guardian.co.uk
The crew maintains a trench latrine, which gets covered with dirt and fire-pit ash.
From the thenewstribune.com
Mortars are able to fire from the protection of a trench or defilade.
From the en.wikipedia.org
Mackay took up a position on a fire step, a raised part of the trench floor which allows men to fire over the top.
From the en.wikipedia.org
The stretcher-bearers in the Argonne, for example, who had told him they didn't dare leave the trench for fear of German fire.
From the economist.com
The trenches connecting the various lines of the besiegers could not be built perpendicular to the walls of the fortress, as the defenders would have a clear line of fire along the whole trench.
From the en.wikipedia.org
They are much wider than normal galleries, suggesting that their primary purpose was to allow circulation of air through the trench and permit safe fire-setting.
From the en.wikipedia.org
The banked earth on the lip of the trench facing the enemy was called the parapet and had a fire step.
From the en.wikipedia.org
Whether the small arms fire had been coming from bunkers in the wall or the trench alongside it, they didn't know.