The CRL Glass Thickness Gauge measures single pane thickness even when it is not possible to measure the glass from the edge. Place the long edge of the Gauge against the glass and tilt the Gauge upward to a 45 degree angle to the glass surface. Look into the glass at the reflection of the Gauge and compare each short line with the long continuous line. Pick the one short line that is shown in the reflection within the same plane as the long line, and read the glass thickness measurementas indicated above that line. This is your glass thickness.