Partial Datum Surface A portion of a surface can be used as a datum. For example, this can be done when a part has a hole or group of holes at one end where it is not necessary to establish the entire surface as a datum to effectively locate the features. This can be accomplished on a drawing using a chain line dimensioned with basic dimensions to show the location and extent of the partial datum surface. The chain line dimension is considered a minimum distance. The datum feature symbol is attached to the chain line. The datum plane is then established at the location of the chain line, as shown in Example 3-24. Chapter 3 Datums 99 The Fixture Setup The Drawing Chain line Datum feature Part Simulated datum (fixture surface) Datum plane Example 3-24. A partial datum surface established with a chain line. Coplanar Surface Datums Coplanar surfaces are two or more surfaces that are on the same plane. The relationship of coplanar datum features establishes the surfaces as one datum plane in correlated feature control frame specifications. A phantom line is placed between the surfaces if a void, such as a slot, exists. The phantom line between surfaces is omitted when the area between the surfaces is higher than the datum features. The surfaces are treated as a single, interrupted surface. The number of surfaces can be specified as continuous by using a note, such as “2 SURFACES,” below the related feature control frame. See Example 3-25A and Example 3-25B. This concept is also discussed in Chapter 9 with an application for profile tolerances of coplanar surfaces.