To properly mount a phono cartridge, you need to be able to position it so that it remains tangent to the record grooves at all diameters. This is always a compromise. This gauge helps you to mount the cartridge to the headshell for the best compromise. This protractor is designed for records from 7 inch to 16 inch diameters. All other gauges are made for 12 inch LP records. Made from sturdy thin gauge cardboard. This instrument is cheap but accurate.