The GD&T feature control frame, explained box by box

A feature control frame is the rectangular box you see attached to a dimension on almost every engineering drawing that carries geometric tolerancing. It looks cryptic the first time: a symbol, a number, maybe a circled letter, then one to three more letters. But it follows a strict reading order defined in ASME Y14.5-2018 and ISO 1101. Once you know the order, every frame on the print reads the same way.

This guide breaks the feature control frame into its compartments, shows you how to read each one, and walks through a real bracket callout. It is written for the design engineer drafting the frame and the quality engineer who has to inspect against it.

A position feature control frame: position symbol, diameter tolerance zone of 0.2 mm at maximum material condition, referenced to datums A, B and C.

What a feature control frame actually controls

A linear dimension like 25.0 ±0.1 controls size. It says nothing about where a feature sits, how straight it is, or how square it is to another face. That is the job of geometric tolerancing. The feature control frame is the sentence that states a geometric requirement: which control applies, how large the tolerance zone is, and what the part is measured against.

Read left to right, every frame answers three questions in the same order: what kind of control, how much tolerance, and relative to what. Get that order fixed in your head and the rest is detail.

Compartment 1: the geometric characteristic symbol

The first box holds one of the fourteen geometric characteristic symbols. This tells you the type of control. The fourteen split into five families:

Knowing the family immediately tells you whether to expect datum letters later in the frame. A flatness callout never has a datum; a position callout almost always does. If you want the full set with shop-floor examples, our guide to the 14 GD&T symbols covers each one in turn.

Compartment 2: the tolerance zone

The middle compartment states the size of the tolerance zone. Three things can appear here, in this order:

1. A diameter symbol (∅) — present only when the zone is cylindrical, as it is for the position of a hole. No diameter symbol means the zone is the gap between two parallel planes or lines.
2. The tolerance value — for example 0.2, meaning a 0.2 mm wide or 0.2 mm diameter zone. This is a total zone, not a ± band. A position tolerance of ∅0.2 gives the axis a cylinder 0.2 mm across to live in.
3. A material condition modifier — an optional circled M, L or nothing. This is where most beginners stumble, so it gets its own section below.

Material condition modifiers: M, L and RFS

The circled letter after the tolerance changes how the tolerance behaves as the feature's size changes:

• Circled M — Maximum Material Condition (MMC). The stated tolerance applies when the feature is at its most material: smallest hole or largest pin. As the hole grows past MMC, you earn extra position tolerance, called the bonus tolerance. This is the right modifier for clearance holes and assembly fits.
• Circled L — Least Material Condition (LMC). The tolerance applies at the least material size. Used to guard minimum wall thickness or minimum edge distance.
• No modifier — Regardless of Feature Size (RFS). The tolerance is fixed and earns no bonus, whatever the feature measures. This is the default in ASME Y14.5 when no symbol is shown.

Compartment 3 onward: the datum references

The remaining compartments hold the datum reference letters — the features the geometric control is measured against. Order is everything here. The first datum letter is the primary datum, the second is secondary, the third is tertiary. They establish the datum reference frame in that sequence and are not interchangeable.

• Primary datum (A): the part is first seated on this feature, removing the most degrees of freedom. Typically the largest, most stable face.
• Secondary datum (B): the part is then registered against this, removing further freedom.
• Tertiary datum (C): the last reference, locking the part fully.

So A|B|C and A|C|B are different requirements and will be inspected with different fixturing. Each datum letter can also carry its own material condition modifier when it refers to a feature of size, which is how you get callouts like position to A, B at MMC, C.

A worked example: locating a bolt hole

Take a 7075-T6 aluminium bracket with a ∅6.0 +0.12/0 mounting hole. The frame reads: position, ∅0.25, circled M, then A, B, C.

On a CMM you would set up the datum reference frame A then B then C, probe the hole axis, and check that it lies within the ∅0.25 zone plus any bonus earned from the measured hole size. The whole callout is a single, unambiguous instruction once the order is understood.

Common mistakes that cause rejections

• Reading ± into the zone. A position of ∅0.2 is a total 0.2 mm diameter zone, not ±0.2. Halving or doubling this is a frequent inspection error.
• Ignoring bonus tolerance. Skipping the MMC bonus rejects parts that are actually in spec, inflating your rejection rate.
• Swapping datum order. Treating A|B|C as if the order does not matter changes the fixture and the result.
• Wrong inspection tool for the control. Position, profile and runout to a datum almost always need a CMM, not a caliper.
• Missing the frame entirely when ballooning. Each feature control frame is a characteristic in its own right and must get its own balloon number on the inspection sheet.

From frame to inspection sheet with CadNexa

Reading the frame is step one. Step two is capturing every frame as a numbered characteristic on a ballooned drawing so it can be measured and signed off. CadNexa's auto-ballooning uses Smart Detect plus Box+Balloon OCR to find dimensions and GD&T feature control frames on a PDF drawing, parse the geometric symbol, tolerance zone and datum references, and place a numbered balloon on each. You review, correct anything the OCR missed, and export a clean inspection sheet. For the full drawing-to-sheet workflow, see how to convert a drawing into an inspection sheet.

Frequently asked questions

What is a feature control frame in GD&T?

A feature control frame is the rectangular box used in GD&T to state a geometric tolerance. Read left to right it holds the geometric characteristic symbol, the tolerance zone value with any material condition modifier, and the datum references. Together these define one geometric requirement on a feature.

How do you read a feature control frame?

Read it left to right in three parts. First the symbol tells you the type of control, such as position or flatness. Second the tolerance compartment gives the zone size, a diameter symbol if the zone is cylindrical, and an optional M or L modifier. Third, any datum letters give the references in primary, secondary, tertiary order.

What does the circled M mean in a feature control frame?

The circled M is the Maximum Material Condition modifier. It means the stated tolerance applies when the feature is at maximum material, and the part earns bonus tolerance as the feature departs from MMC. It is commonly used on clearance holes to capture extra, legal position tolerance.

Does the order of datum letters matter?

Yes. The first datum letter is primary, the second secondary, the third tertiary. They build the datum reference frame in that order and constrain the part step by step, so A|B|C and A|C|B describe different fixturing and different requirements.

How is a feature control frame inspected?

Set up the datum reference frame in the stated order, then measure the controlled feature against the tolerance zone. Location, orientation, profile and runout controls referenced to datums are usually measured on a CMM, while simple form controls can sometimes be checked with surface plates and indicators.

By Rajadurai R — Founder, 14 years plant-head experience. For more tutorials, visit the CadNexa learning center.