What is a Datum Feature?

In Geometric Dimensioning and Tolerancing (GD&T), a Datum Feature is an actual feature on a part—such as a surface, hole, or slot—that serves as a reference for measurement and geometric control. It is identified on drawings using the Datum Feature Symbol, a rectangular frame with a capital letter (e.g., A, B, C).

Datum features provide the foundation for creating a datum reference frame, which is the coordinate system used to control orientation and location of other part features.

How Datum Features are Used

• Establishing reference frames – Primary, secondary, and tertiary datums define a 3D coordinate system for inspecting and manufacturing parts.
• Controlling orientation – A datum surface may be used to control perpendicularity, parallelism, or angularity of other features.
• Controlling location – A datum hole or slot may be used to define the true position of other features relative to it.
• Ensuring interchangeability – Datums guarantee consistent measurement and functional fit across multiple manufactured parts.

Example of a Datum Feature

Imagine a rectangular block where one large face is identified as Datum Feature A. This surface acts as the primary reference plane. A perpendicular side face may be assigned as Datum Feature B, and a hole through the block could be assigned as Datum Feature C. Together, A-B-C form the datum reference frame used to inspect the part.

Why Datum Features Matter

Datum Features are critical in GD&T because they establish the framework for interpreting all other geometric tolerances. Without clear datum references, measurement results can vary significantly and lead to part misalignment or assembly issues.

By correctly applying datum features, engineers ensure design intent is preserved, inspection is repeatable, and manufactured components fit and function as intended.