Flatness tolerances control the divergence or departure of a surface from a true plane.
The tolerance of flatness is the specified zone between two parallel planes. It does not control the squareness or parallelism of the surface in relation to other features, and it can be called for independently of any size tolerance.
Drawing instruction Note that, at any particular section, a circle may not be concentric with its axis but may still satisfy a circularity tolerance. The following diagram shows a typical condition.
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