The inboard ribs are not match-drilled, so we had to be sure and place them properly in relation to the forward and aft spars, then clamp them. The skin has pre-drilled holes, so I could mark through the holes onto the ribs' flanges with a sharpee, mark a centerline reference on those flanges, then reposition the ribs and clamp them again. With small adjustments I found those reference marks and started drilling......
