Modulus of Elasticity (E) measurements have shown that stiff surfaces attached to softer substrates deteriorate rapidly in response to cyclical changes in temperature and relative humidity. Conversely, a softer outer layer firmly attached to a stiff substrate withstands environmental influences better, as expressed by the old painters' rule of Fat Over Lean. In canvas paintings, this relationship becomes reversed during the aging process. With aging, the imbalance between the hardening paint layer and the concurrent loss of tension and stiffness in the canvas keeps growing. This imbalance makes reinforcement to stiffen the canvas inevitable, to preserve the image of the painting. Protective lining might be preferable to progressive damage resulting from neglect, or to treatments which have no effect on the main causes of deterioration.