It is a well-known fact that jacket crowns sometimes fracture from apparently nor mal use. The types of fractures— the longitudinal split of the crown and the crescent-shaped fracture in the labiogingival or linguogingival regions ■— are familiar to all dentists. The purpose of the present study was to determine the causes of such fractures by analyzing the regions of strain within jacket crowns subjected to a load. The external shape of the jacket is determined by tooth form and physio logical requirements, but the core or sup porting tooth substance can be modified within certain limits to achieve two essen tial requirements: stability and strength. I f the crown is to have stability, it must not rotate, twist or deform excessively under tooth loads. Stresses in the crown must be low enough to prevent fracture due to tooth loads. Obviously, the core design must be a compromise between the shapes de manded by the two requirements of sta bility and strength. From the standpoint of stability, a cross section of the core should be rectangular with sharp corners; but from the standpoint of strength, a cross section of the core should be round or oval-shaped or at least rectangular with generously beveled or rounded line angles. It was decided, therefore, that if a large plastic prototype of a jacket was loaded on an appropriate core and then sectioned, it would be possible by view ing the sections under polarized light to find the precise regions of detrimental strains. Certain changes in the shape of the core could then be made to reduce these strains and thus reduce the inci dence of fracture of jacket crowns.