Background 

Oriental rugs are woven from bottom to top.  The weaver ties individual knots to comprise the pile of a rug.  A knot is woven in a downward motion and "opens" down, left or the right (depending on loom offset and type of knot) which creates a pile direction.  Varying from rug to rug, style of weaving, and type of knot, the rug may have a distinct pile direction.  This direction can be felt as well as seen.  If you take your hand to the pile of the rug, you may find that running your hand vertically (direction of fringe to fringe) or diagonally, you will feel how the pile has both a rough and smooth side.  Running your hand against the pile is termed going "against the pile direction."  Running your hand with the smooth side is termed "with the pile direction."

Why does the rug have a light and dark side?

The rug's light and dark sides result, in part, from the pile direction.  Some carpets may have more pronounced light and dark sides than others, depending on the weaving style and the lustre of the fiber.  A more technical analysis follows:

Pile fibers lay at an angle where they will do one of two things: reflect or absorb light.  On the rug's lighter side, light reflects off the fiber's sides, giving the pile a brighter sheen.  From the dark side of the rug, your view is the cut tips of the wool which absorb light rather than reflect it, appearing to give the rug a deeper tone.

This is the Light side of an Oriental Rug

This is the Dark side of an Oriental Rug

Looking With the Pile (Light Side)

Light is Reflected

Looking Into the Pile (Dark Side)

Light is absorbed