16 Geometric Dimensioning and Tolerancing Copyright Goodheart-Willcox Co., Inc. Dimension and Tolerance Origin Coordinate tolerancing does not offer a standard method for defining the origin of dimensions. It is unclear if, for example, dimensions measure from the right to left or left to right. It is also unknown if the start point of a dimension is on a reference line, plane, or point, or on the surface of the feature. Example 1-12 shows some of the many options for interpreting the 15±0.25 dimension that locates the hole in the face plate using coordinate tolerancing. The options are illustrated using magnified views of the manufactured part to show exaggerated uneven form and orientation of the surfaces. You will see similar representations throughout this textbook. Notice in Example 1-12 how variations in the surface form and orientation affect measurements. A measure- ment taken from a point on a surface differs from a measurement taken from a refer- ence plane. Goodheart-Willcox Publisher Example 1-11. Constraining the face plate in a set of three mutually perpendicular intersecting datum planes ensures a common basis for measurements and for inspecting every part the same way every time. Measurements are taken perpendicular or parallel to the datum planes.