Before putting the aluminum skins on the wings, the frame has to be perfectly square. The spars (the green things at the front and back of the wing) have to be parallel and there can be no twist in the spars. Both ends have to be perfectly vertical. I probably spent the better part of a week trying to get everything straight. Here, I am using a digtal level to see if the bottom spar is level at all points. In the back, I have attached some waits to straighten out a little twist. The tape measure attached to the bottom spar assures that each rib is at the right place as measured from the center of the airplane (called butt length).