A feature control frame is the rectangular boxed callout used in GD&T. It is read from left to right: the geometric characteristic comes first, followed by the tolerance and any modifier, followed by datum references when the control needs them.
Cell one: the geometric characteristic
The first compartment contains the GD&T symbol. It identifies whether the requirement controls form, profile, orientation, location, or runout. The leader, extension line, or placement of the frame tells you which surface, axis, or center plane the control applies to.
The tolerance cell: size and shape of the zone
The next compartment gives the tolerance value. A diameter symbol means the tolerance zone is cylindrical; without it, the zone may be two parallel planes or a profile zone, depending on the control. A circled material-condition modifier such as MMC or LMC changes how the tolerance relates to feature size.
Datum references: the inspection setup
Datum letters on the right identify the datum reference frame. They are read in order: primary, secondary, then tertiary. The sequence tells the inspector how to orient and locate the part before measuring the controlled feature. Form controls such as flatness commonly have no datum reference.
Read a position callout
This frame calls for a 0.20 diameter position tolerance at maximum material condition, evaluated from datum reference frame A-B-C. See the complete feature control frame guide.
Frequently asked questions
Feature Control Frame Explained: How to Read GD&T Callouts FAQ
What is the first symbol in a feature control frame?
It is the geometric characteristic symbol. It identifies the type of geometric requirement, such as flatness, position, perpendicularity, or profile.
What do the letters A, B, and C mean?
They are datum references. In order, they establish the inspection setup used to evaluate the controlled feature.
Does every feature control frame need datums?
No. Form controls, including flatness and circularity, are self-contained and normally do not reference datums.
Keep reading
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