The GD&T Guide
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Foundation 02

Feature Control Frames

The boxed instruction that tells you what is controlled, how much variation is allowed, and what it is referenced to.

A feature control frame is the compact language of GD&T. Read it left to right: first the geometric characteristic, then the tolerance and any modifier, then the datum references that establish the inspection setup.

The first compartment: what is controlled

The first cell contains the geometric characteristic symbol. It tells you whether the requirement is about form, profile, orientation, location, or runout. The leader or association shows which feature it applies to.

The tolerance compartment: how much variation is allowed

The next cell gives the tolerance value and may include a diameter symbol or a material-condition modifier. Together, these tell you the size and shape of the tolerance zone—not simply a plus/minus dimension.

Datum references: how the part is set up

The cells on the right list datum references in order of precedence. They tell an inspector how to orient and locate the part before evaluating the controlled feature. Form controls often have no datum cells because they are self-contained.

Example feature control frame

⌖⌀0.20MABC

This example controls a hole axis inside a 0.20 mm diameter position zone at MMC, located from datum reference frame A-B-C.

Common mix-up

Do not read a feature control frame without checking the feature it is attached to. The same symbol can create a different zone for a surface, an axis, or a center plane.

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Foundation 02

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