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Basic dimensions define perfect geometry. Basic dimensions may be used to define the perfect form and size of individual features, and they may be used to define perfect geometric relationships, such as the perfect orientation and location between features. Basic dimensions are also used to define datum targets and the extents of specifications, such as between (e.g. A↔B) and limited areas on parts. A rectangular frame is placed around a basic dimension value. Basic dimensions may be explicitly defined (displayed) on a drawing or an annotated model and basic dimensions may be implicitly defined, such as by stating that all dimensional values obtained from a 3D CAD model are basic. Remember, basic 90° angular dimensions do not need to be displayed on drawings because of the fundamental rules in ASME Y14.5.
The arrangement of basic dimensions is less important than the arrangement of directly-toleranced dimensions (+/-). Arranging directly-toleranced dimensions differently changes tolerance requirements, because directly-toleranced dimensions include tolerances. Arranging basic dimensions differently does not change tolerance requirements, because basic dimensions define perfect geometry – basic dimensions do not include a tolerance. Thus, the dimensioning methods shown in the following figure are equivalent.
Figure 1 – Two Ways to Dimension Relationships Between Features