Flat-front trousers have no pleats. This style means the trouser lies close to the body without any extra material around the lower torso; by contrast, pleated trousers on a man’s suit accommodate some extra material to allow more room and more movement.
Flat front trousers have a snug, slimming fit that preserves the long, straight line usually regarded as desirable particularly in European style men’s suits.
On larger men, the flat front trouser can seem too tight and could even be uncomfortable to wear.