How is the datum symbol indicated on a drawing, and what does it convey?

How is the datum symbol indicated on a drawing, and what does it convey?

• A. A circled letter (A, B, C) indicates the datum feature; it conveys which surface or axis sets the reference for tolerances
• B. A triangle marks datum; it conveys process capability
• C. A square box indicates the datum; it conveys material type
• D. An arrow marks the datum; it conveys alignment

Answer: (A)

Datums establish the reference frame used to interpret geometric tolerances. On a drawing, the datum is indicated by a circled capital letter placed on the feature being used as the reference, with a leader line pointing to that surface or axis. This circled letter identifies which feature provides the origin for measurements, and the tolerances in the feature control frames are evaluated relative to that datum. If multiple datums appear, their order defines the primary, secondary, and (if present) tertiary datums, which sets how the part is oriented and located during inspection. This communicates the intended origin and orientation for all tolerances, not material type or process capability. Other symbols like triangles, squares, or arrows are not the standard way to designate a datum in GD&T.

Datums establish the reference frame used to interpret geometric tolerances. On a drawing, the datum is indicated by a circled capital letter placed on the feature being used as the reference, with a leader line pointing to that surface or axis. This circled letter identifies which feature provides the origin for measurements, and the tolerances in the feature control frames are evaluated relative to that datum. If multiple datums appear, their order defines the primary, secondary, and (if present) tertiary datums, which sets how the part is oriented and located during inspection. This communicates the intended origin and orientation for all tolerances, not material type or process capability. Other symbols like triangles, squares, or arrows are not the standard way to designate a datum in GD&T.