Flatness (⏥)

How to read it

The flatness symbol is a parallelogram leaning to the right, in the first compartment of a feature control frame followed by the tolerance. The National Physical Laboratory’s guide defines the zone as two parallel planes, and NIST’s Vijay Srinivasan reads an ISO flatness frame of 0.12 the same way: points taken from the real surface have to fit between two parallel planes 0.12 mm apart. ASME Y14.5-2018 puts flatness among the form controls, with no datum reference (Berez, UNC Charlotte). Cogorno’s course guide adds that the flatness zone doesn’t have to be parallel to any other surface of the part.

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Look-alikes

Parallelism ∥ also has a zone between two parallel planes, but those planes are held parallel to a datum, and its frame ends in a datum letter. Berez’s notes put it in one line: parallelism is not flatness, because it has a datum reference. Unicode encodes the flatness parallelogram as ⏥ (U+23E5) FLATNESS.

Flatness under ASME and ISO

Under ASME Y14.5 the limits of size of a feature of size also control its form (Rule #1), so on a plate the flatness tolerance is a refinement inside the size limits; Cogorno puts it as the surface lying in the flatness zone while the feature also falls within its size tolerance. ISO drawings follow ISO 8015’s principle of independency instead. Keyence’s comparison of the two standards shows the same flatness callout on a plate read both ways: under ISO the flatness stays within its stated value whatever the size tolerance allows, and under ASME the envelope principle applies and the flatness allowed changes with the material condition.