18 Geometric Dimensioning and Tolerancing Copyright Goodheart-Willcox Co., Inc. Datums specified on a drawing or model defined with GD&T establish the dimen- sion origin. All measurements originate from and are taken perpendicular or parallel to the referenced datums. Example 1-13 shows how the 15 basic dimension locating the hole originates from and is perpendicular to datum plane B. A basic dimension is a dimension that is considered theoretically perfect in size, profile, orientation, or location. Positioning the part using a reference coordinate system of datums ensures a common basis for measurements and for inspecting every part the same way every time. Tolerance Zones Unlike coordinate tolerancing, GD&T uses geometric tolerances to control specific features or elements of features. Geometric tolerance zones established with geomet- ric tolerances in GD&T provide greater precision in defining part requirements and greater flexibility in manufacturing. In coordinate tolerancing, tolerances directly applied to dimensions define allowable size or location variations. Example 1-14 shows the allowable limits associated with the 16±0.5 dimension between two sur- faces of the face plate. Any local measurement between the high points of the surface profile cannot exceed the maximum limit of 16.5, and any local measurement between the low points of the surface profile cannot be less than the minimum limit of 15.5. Feature in the Orthographic View Meaning 15 From datum feature B to the center of the hole Origin here from datum plane B Datum feature symbol Secondary datum feature (datum feature B) 90° Datum plane B 15 0.7 Diameter cylindrical tolerance zone at MMC Goodheart-Willcox Publisher Example 1-13. Positioning the part using referenced datums allows measurements to be taken from and perpendicular to datum planes.