The perpendicularity between the hinge line and midrib in the undeformed state is lost. This also helps to find the shear strain. Originally equilateral shells become inequilateral and these in the 19th century gave spurious species of lamellibranchs or pelecypods in the Alps.
The 120 degree intersections between adoining walls are changed. The angle gets reduced in the extensional zone but increased in the contractional zone of the finite strain ellipse.