What is the significance of circled letters in GD&T callouts?

• A. They denote datum identifiers used to establish the DRF.
• B. They indicate material condition flags
• C. They mark features to be machined
• D. They identify surface finishes

Answer: (A)

Circled letters identify datum references that establish the datum reference frame used to evaluate the part. In GD&T, datums define the theoretical origin, orientation, and location for measuring tolerances. The circled letters point to the specific datum features (A, B, C, etc.) that form the primary, secondary, and tertiary datums, and they appear in the datum references within the feature control frames. This labeling tells you exactly which features are used to create the coordinate system against which a tolerance is checked. It’s not about machining steps, material conditions, or finishes—the circled letters simply denote the datums that anchor how the tolerance is applied.