GD&T Measuring Instruments and Principles
While size features only concern the measurement of the linear distance between two points, geometric features concern the characteristics of form, such as roundness, straightness, and curvature, which makes measurement with measuring instruments that measure the distance between two points (e.g. calipers and micrometers) difficult.
There are measuring instruments and machines to measure features for GD&T measurement. The accuracy, operability, and prices of such instruments and machines vary.
Instruments and machines that measure geometric characteristics can be categorized as follows. In this section, we will introduce the instruments and machines that are suitable for measuring geometric characteristics.
 Measuring Instruments

Surface plate, dial gauge, V block, height gauge, limit gauge/function gauge (location gauge)
 Measuring Machines

Roundness measuring machine, coordinate measuring machine (CMM)