What is a Datum?

On any given part, there are three datums (A, B, and C) that are aligned with the primary, secondary, and tertiary planes of a datum reference frame (more on that later). 

Datums are determined by selecting specific features on the part. While these features are based on datum planes, they do not always have to be planes themselves.

Datum features are based on points of contact and are ordered according to the degrees of freedom that physically constrain the part (critical for inspection purposes). This is commonly referred to as the 3-2-1 rule, which defines the number of points of contact required to establish the primary, secondary, and tertiary datum planes. 

Datum A

This is the primary datum, as it corresponds to the primary plane. Datum A is the first to make contact and requires at least three points of contact. In the example below, Datum A constrains the Z axis, or translation, and the U and V rotations along the X and Y axes, respectively.

Datum B

Think of this as the secondary datum, which corresponds to the secondary plane. Datum B requires two points of contact and constrains the Y axis and the W rotation along the Z axis.

Datum C

This functions as the tertiary datum and corresponds to the tertiary plane. Datum C is the least important, although not irrelevant, of the datums and has only one point of contact. In the example below, it only constrains the X axis.

How to Choose the Right Datum

A well-selected datum mirrors how the part interfaces in the final assembly, represents critical functional surfaces, and provides stable, repeatable inspection setups, and a well-designed DRF replicates how the part behaves in the assembly. 

For example:

• A mounting plate → primary datum = mounting surface
• A shaft → primary datum = axis of rotation
• A housing → primary datum = interface face

This alignment ensures that tolerances directly control functionality, inspection replicates real-world use, and suppliers don’t “pass bad parts” that fail in assembly.

A well-chosen datum solves three persistent engineering problems:

1. Ambiguity in drawings – GD&T and datums provide a common language across design, machining, and quality teams
2. Inconsistent inspection results – the DRF defines exactly how to orient and measure the part, eliminating interpretation
3. Assembly failures – datums tie tolerancing to function, ensuring the part fits, aligns, and performs under load

What's the Difference between a Datum and a Datum Feature?

Before diving into the details, let’s tackle a common misunderstanding – the difference between a datum and a datum feature.

Datum

A datum is a theoretically exact point, axis, line, plane, or combination that’s derived from the theoretical datum feature simulator. The most important thing to remember about datums is that they are theoretically exact.

Datum Feature

A datum feature is a real part feature (e.g., face, edge, hole, etc.) that’s identified with either a datum feature symbol or a datum target symbol. Datum features are not theoretical.

This distinction matters because manufactured parts are inherently imperfect. Datums allow you to compare imperfect reality to a theoretically perfect model, and datums exist within the datum reference frame.

What is a Datum Feature? 

A datum feature is a specific part feature, such as a face or an edge, that's identified with either a datum feature symbol or a datum target signal. These features are associated with real parts and are not theoretical. 

This table lists the different datum features, how they constrain degrees of freedom, and how a datum feature callout is shown on a technical drawing.

What is a Datum Reference Frame?

A datum reference frame (DRF) is three mutually perpendicular intersecting datum planes that serve as the reference for a component's location, orientation, and form relationships. The datum planes constrain how your part is machined, inspected, and assembled.

The DRF establishes a set of orthogonal planes that are used by all subsequent feature controls and tolerances specified on a part. It provides a common reference point for all dimensions and tolerances, ensuring consistent interpretation and application of GD&T symbols and tolerances. 

Your DRF and datums define the degrees of freedom and constrain the part, and the sequence of datum features you choose directly relates the part to the DRF. This is commonly referred to as the 3-2-1 rule, which defines the number of points of contact required to establish the primary, secondary, and tertiary datum planes. These planes constrain the degrees of freedom in translation (x, y, and z axes) and rotation (u, v, and w rotations).

A properly constructed DRF must contain these degrees of freedom in a defined sequence:

Datum A

Datum A is the first to make contact and requires at least three points of contact. In the example below, Datum A constrains the Z axis, or translation, and the U and V rotations along the X and Y axes, respectively.

Datum B

Think of this as the secondary datum, which corresponds to the secondary plane. Datum B requires two points of contact and constrains the Y axis and the W rotation along the Z axis.

Datum C

This functions as the tertiary datum and corresponds to the tertiary plane. Datum C is the least important, although not irrelevant, of the datums and has only one point of contact. In the example below, it only constrains the X axis.

How to Choose your Datums

The most practical thing to know about datums is that they must reflect function—not convenience. Before discussing datum selection strategy, let's walk through the types of datums that are at an engineer's disposal. 

A well-selected datum mirrors how the part interfaces in the final assembly, represents critical functional surfaces, and provides stable, repeatable inspection setups, and a well-designed DRF replicates how the part behaves in the assembly. 

For example:

• A mounting plate → primary datum = mounting surface
• A shaft → primary datum = axis of rotation
• A housing → primary datum = interface face

This alignment ensures that tolerances directly control functionality, inspection replicates real-world use, and suppliers don’t “pass bad parts” that fail in assembly.

Engineering Takeaways 

With regard to datums and DRFs, the selection is less about what's allowed and more about what reflects functional assembly conditions:

• Use planar datums for stability and fixturing fidelity
• Use axes when rotational alignment or concentricity drives function
• Use median planes when symmetry matters (e.g., slots, tabs) 
• Avoid abstract datums (e.g., points or lines) unless absolutely required