Servos have an output shaft that typically uses a splined profile in order to transfer the torque from the output shaft into the servo attachment that is fastened to it. This splined profile is typically classified by the number of teeth, however, there is potential for two very different servo splines to have the same tooth count as it doesn't capture the spline diameter or tooth profile. For example, A15T and D15T both have 15 teeth, however the D15T is a much larger spline size than the A15T. The graphics below are loose representations of the common spline sizes that are found on hobby servos. If you have a servo that we do not offer on the site, you can check to see if your servo has a spline that matches any of the below spline profiles by counting the teeth and measuring the distance across the spline.