Reference dimension


A reference dimension is a dimension on an engineering drawing provided for information only. Reference dimensions are provided for a variety of reasons and are often an accumulation of other dimensions that are defined elsewhere. These dimensions may also be used for convenience to identify a single dimension that is specified elsewhere.
Reference dimensions are not intended to be used directly to define the geometry of an object. Reference dimensions do not normally govern manufacturing operations in any way and, therefore, do not typically include a dimensional tolerance. Consequently, reference dimensions are also not subject to dimensional inspection under normal circumstances.
Prior to use of modern Computer-Aided Design software, reference dimensions were traditionally indicated on a drawing by the abbreviation "REF" written adjacent to the dimension. However, standard ASME Y14.5 has changed the way references are marked and the abbreviation "REF" has been replaced with the use of parentheses around the dimension. As an example, a distance of 1500 millimeters might be denoted by "" instead of "1500 mm REF." This implementation has followed in modern CAD software that makes use of parentheses as the default denotation method whenever reference dimensions are "automatically" created by the software.