What is "opus latericium"

thumb|250px|Example of opus latericium on a tomb of the ancient Appian Way in Rome. Opus latericium ( Latin for "brick work") is an ancient Roman form of construction in which coarse-laid brickwork is used to face a core of opus caementicium.

Opus latericium was the dominant form of wall construction in the imperial era. In the time of the architectural writer Vitruvius, opus latericium seems to have designated structures built using unfired mud bricks.