Tertiary Datum – Oriented perpendicular to both the primary and secondary datum and located on a surface relative by translation to the location of the form tolerance zone. Secondary Datum – Oriented perpendicular to the primary datum with similar location controls. Primary Datum – Datum feature oriented with respect to the location of the form tolerance zone (or the surface where the feature is controlled) and relative to the surface making up the geometric feature the datum is taking form of, such as a plane. The primary, secondary, and tertiary reference datums control the feature by fully defining the location and orientation. Each part or feature is controlled by a coordinate system that introduces degrees of freedom onto the feature. Dimensions and geometric tolerances can reference datums, indicating that the measurement is with respect to that reference frame. The datum is a reference point, surface, or axis on a part that controls the geometric feature measurements are made from.
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