Flatness represents the geometric concept of the plane, when applied to a solid surface where it becomes measurable by mechanical means, either directly or in combination with optical instruments.

Flatness is a functionally meaningful condition in many technical parts for reasons related to the intended service of that part. Controlled flatness may be required to provide full contact with a mating part, such as in seals; as a precondition for the parallelism of nominally flat surfaces; as a reliable boundary plane for linear dimensions; to provide locating planes for the dependable mounting or assembly of manufactured parts; and many others.

Considering the functional role of flatness, the measurement of that condition on the operating surfaces of manufactured parts is often an important operation of the dimensional inspection process. The choice of the best suited method of flatness measurement is governed by various factors, such as the size and the shape of the part; the area to be inspected; its accessibility and interrelations with other surfaces; and the desired degree of measuring accuracy.

Table 11-3 presents examples of different methods of flatness measurement on technical surfaces; these encompass a wide range of instrument categories and degrees of sensitivity. Although the listing comprises methods from the shop level to the most accurate laboratory procedures and instruments, its purpose is to indicate the variety of possible choices, rather than to attempt completeness.