This spacer is used with carbon screen-printed electrodes (SPEs). The Screen-Printed Electrode Grip/Mount is designed for use with carbon, platinum, and gold SPEs.  Carbon SPEs are 0.36 mm thick whereas Platinum and Gold SPEs are 0.67 mm thick.  Therefore, when using carbon SPEs with the grip, you must increase the thickness by adding a spacer on the back side of the SPE, such that it fits in place without falling out of the grip.  These spacers are included with the purchase of any carbon SPE.