Communication trenches linked the front line to the support trenches during World War I. These narrow passageways ran roughly perpendicular to the main trench lines, allowing soldiers, supplies, stretcher-bearers, and messengers to move between positions without exposing themselves to enemy fire above ground. They formed the connective tissue of the entire trench system, running not just between the front and support lines but extending further back to reserve trenches, artillery positions, and command posts.
How Communication Trenches Worked
A typical trench system on the Western Front consisted of three parallel lines: the front-line (or fire) trench facing the enemy, a support trench roughly 100 to 200 yards behind it, and a reserve trench further back still. Communication trenches connected all three, creating a network that soldiers could navigate entirely below ground level. The French called them “boyaux,” meaning guts or tubes, which captures their function nicely.
Standard British communication trenches were about 5 feet wide at the top and 5 feet deep, slightly narrower than fire trenches. A wider variant measured 8 feet across and 6 feet deep, designed for heavier traffic. The bottom was typically around 2 feet wide, just enough for one person to pass through. These dimensions meant that two soldiers carrying a stretcher could barely squeeze by, and moving large groups through them was slow, exhausting work.
Why They Were Built in Zigzags
Communication trenches were never dug in straight lines. They followed zigzag, crenellated, or wavy paths, and this design choice was primarily about surviving artillery. A shell landing in a straight trench would send its blast wave and shrapnel tearing along the entire length, killing or wounding soldiers for dozens of meters in both directions. Square corners and sharp turns broke up those shock waves, containing the damage from a direct hit to a small section of trench. If a shell landed in one segment, the soldiers around the next corner were largely protected.
This also mattered if the enemy ever captured part of the trench system. A straight communication trench would give an attacker a clear line of fire all the way back to the support or reserve positions. Zigzag construction meant that even an enemy standing inside the trench could only see and shoot as far as the next bend, buying defenders time to organize a counterattack.
What Moved Through Them
Communication trenches carried everything the front line needed to function. Fresh troops moved forward through them to relieve exhausted units. Ration parties hauled food, water, and ammunition up to the fire trench under cover of darkness. Wounded soldiers were carried back on stretchers to dressing stations in the rear. Runners carrying written messages sprinted through them when other forms of communication failed, which happened constantly.
They also served as conduits for communication infrastructure itself. Telephone wires and buried cables ran through or alongside communication trenches, connecting forward observation posts to artillery batteries and command centers. The British and Germans invested heavily in deep-buried cable networks with underground junction boxes spaced every few hundred yards. The French took a different approach, stringing their telephone lines on wooden supports set against the walls of open trenches. Either way, the communication trench was the physical pathway that made coordinated warfare possible across a sprawling defensive system.
Traffic and Congestion
With a bottom width of only about 2 feet, communication trenches created serious bottlenecks. Units moving forward to the front line would meet stretcher parties, supply carriers, and messengers heading the other way. In some sectors, specific communication trenches were designated as “up” or “down” routes to manage the flow, much like one-way streets. Passing bays, small widened sections cut into the trench wall, gave soldiers a place to press aside and let others squeeze past.
Despite these measures, congestion was a persistent problem, especially during relief operations when hundreds of men needed to move through the system at once. Rain turned the narrow floors into knee-deep mud, slowing movement to a crawl. A journey of a few hundred yards could take an hour or more under bad conditions, all while under the threat of shellfire targeting the known routes.
Scale of the Network
The sheer extent of these connecting trenches is easy to underestimate. Archaeological surveys of former Western Front positions have mapped roughly 420 kilometers of combined fire and communication trenches in single study areas alone. Across the entire front, thousands of kilometers of communication trenches existed at any given time, constantly being destroyed by shelling and rebuilt by working parties. Maintaining them was one of the most grueling and dangerous routine tasks of trench warfare, consuming enormous amounts of labor every night.