Toggling On/Off the Ruler's Calipers
How do I turn the calipers on the Ruler tool on and off?
The Ruler tool's calipers can be toggled on and off from the Ruler's menubar item. With the tool open, select Rulers in the menubar and select "Display Calipers" to turn this feature on and off. You can also use the caliper toggle control, as seen in the image below.