Calipers, vernier caliper

 A caliper is a device that is used to  measure distance between two opposite sides of an object. 

Types

Many types of calipers permit reading out a measurement on a ruled scale, a dial, or a digital display. 

But a caliper can be as simple as compass with inward or outward-facing points. 

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The tips of the caliper are adjusted to fit across the points to be measured and then the caliper is then removed and the distance read by measuring between the tips with a measuring tool, such as ruler