Balanced section calculations assume that the section has been deformed by a plane strain. If the section is underlain by a décollement, it can be restored to its predeformational length by dividing the area of the section by its original stratigraphic thickness. The thickness can be measured in undisturbed foreland beds or reconstructed by unstraining deformed beds. The restoration of the balanced section does not depend on any particular mechanism of deformation or folding, but in the special case of flexural slip folding the section can be unstrained by straightening out the sinuous bed lengths of the folds. However, in nature the area of the section may have decreased by 15–45%, but the assumption of plane strain always leads to a minimum estimate of shortening. Balanced section calculations suggest that the margins of orogenic belts have contracted by 35–54%.