the covering material shrinks more in the longest direction...perpendicular to the roll. When you cover a wing, roll out the covering with the roll parallel to the fuselage and cut your wing-panel covering from the sheet like this. If you turn the sheet sideways, it shrinks front-to-back and dips deeply between the ribs. If you do it the right way, it shrinks along the span and dips much less between the ribs.

It makes a REALLY big difference on a wing that has an unsheeted front section.