Folding into thirds

It's easy enough to fold a piece of paper in half.  Just fold one end onto the other.  But whenever I fold paper into thirds, I just guess where one third is and fold it there.

But there is a way to fold it into thirds exactly.  Can you figure it out?

(This is a relatively easy puzzle inspired by my recent attempts at origami. In the mean time, I've been trying to figure out if there is a way to fold an exact regular pentagon.  It seems that the standard way is to approximate.)

See the solution