The document discusses how to use a Vernier caliper to accurately measure diameter, thickness, depth, and other linear measurements. It explains that a Vernier caliper has inside jaws, outside jaws, a Vernier scale, main scale, and depth gauge. It provides instructions for measuring internal diameter, external diameter, and depth using different parts of the caliper. It also describes how to obtain an accurate reading by matching lines on the Vernier and main scales.