Short definition
The orientation of the carpet pile (the upright fibres) — which way it lies determines how light reflects off the surface and how rooms look together.
Pile direction is the orientation of the upright fibres in a carpet. On any cut piece, those fibres lie at an angle that catches the light one way looking down the run and another way looking against it. The same carpet looks two shades darker against the pile than with it — the effect is real even on plain solid-colour broadloom.
In a flooring takeoff, pile direction matters because adjoining rooms typically need their pile running the same way to look uniform from a shared hallway. A bedroom with pile running north-south sitting next to a living room with pile running east-west is visible to the naked eye through the doorway, and customers notice. The lay-direction decision has to happen before the rolls are cut, not after.
The constraint is most visible on loop-pile carpet and on long hallways, where any pile mismatch shows from a distance. Cut pile is more forgiving but still has the issue at the seam between two rooms with different orientations. The conservative default: lay all rooms with the pile running in the same direction relative to the main entry of the house.
Pile direction interacts with the broadloom roll-width problem. A 5 m wide room cut from a 3.66 m roll forces two drops; running both drops with the same pile direction uses more linear metres than alternating them, but alternating produces a visible seam at the join. The decision usually goes for matched pile and the extra metres get absorbed into wastage.