A Master probe is the probe with which your MicroScribe arm was calibrated. It is identified by a red ring or dot on the neck of the probe. The Working probe looks just like the Master probe, but without the red ring or dot. It is intended for regular use. The Master probe is not for everyday use and should be stored in a safe place until needed. It should be used for calibrating your Working probe and for conducting the Single Point Accuracy Test (SPAT) with your MicroScribe. In the event your device needs service, your Master probe should be returned to Revware for use in testing and/or re-calibrating your device.