Calipers

Calipers are tools used (like Gauges) to take or transfer distance measurements to compare them or read their value off a Rule or similar device.

Outside Calipers	Used to transfer the outside diameter of a bar or thickness of a plate for example, to a rule where it can be measured or to compare it with another part of a project (e.g. to test if a spindle has been turned to a particular diameter or to check if it will fit into a given component). They are available as a firm joint or spring joint version (which is preferred where accuracy is required).
Inside Calipers	These also are available as firm joint or spring joint calipers and are used to check or transfer internal measurements (e.g. to check the width of a slot or inside diameter of a bore).
Oddleg Calipers	Usually supplied with a firm joint, Oddlegs are most used to scribe a line at a set distance from an edge.
Vernier Calipers	These can be used for accurate measurement of either inside or outside dimensions. Accuracy in use depends on closing the caliper with just the right amount of ‘feel’ or pressure, and reading the value off the Vernier Scale - although versions are available which show the values on a dial or provide a digital read out.
Micrometer Caliper	A micrometer is essentially a very accurately machined nut and bolt such that a single turn will equal the pitch of the screw, which is usually half a millimetre. The barrel is divided into 50 and so a single division is equal to a fiftieth of a revolution or a hundredth (0.01) of a millimetre. Outside, Inside and Depth reading versions are available each with options of dial or digital read outs.