Caliper rule - Slide caliper

Slide caliper can be used for measuring outside and inside dimensions. Graduations are in inches, fractions, or millimeters.  The Slide caliper is made as a separate rule or it is incorporated in a folding rule like the extension on a extension rule.
 
Caliper rule - Slide caliper
 
1. Slide
2. Line for internal measurement
3. Clamp nut
 
To measure the inside diameter of a hole, or distance between two surfaces, insert only the rounded tips of the caliper jaws into the hole or between two surfaces. Read the measurement.

 Measuring the inside diameter with a caliper ruler

To measure the outside diameter of round stock, or the thickness of flat stock, move the jaws of the caliper into firm contact with the surface of the stock. Read the measurement.

 Measuring the outside diameter with a slide caliper
 
The Slide caliper should not be used to measure the outside diameter of a cylinder having a radius greater than the depth of the caliper opening.
 
Slide caliper opening