Screw threads for mechanical fasteners have been studied extensively over the years by researchers in many nations. Present commercial standards for metric threads have not incorporated any of the potential improvements. The American National Standards Institute Special Committee to Study Development of an Optimum Metric Fastener System studied and evaluated many means of improving screw threads. It has developed a modified ISO thread profile which improved fatigue resistance but which functionally assembled with existing ISO threads. The thread is defined by boundary profiles for gages to minimize confusion in defining acceptability conditions. A new gaging system based on assembleability and control of ridge width for thread strength has been developed.