In design engineering, size is used for purposes of general identification; the actual size of a part will
be approximately the same as the nominal size but need not necessarily be exactly the same. For ex-
ample, a rod may be referred to as 1/4 inch, although the actual dimension on the drawing is 0.2495 inch;
in this case 1/4 inch is the nominal size.
4.7 TOLERANCES.
The phrase “geometric dimensioning and tolerancing” (GDT) refers to the allowable variation of a dimen-
sion. It represents the difference between the maximum and minimum acceptable limits. All dimensions
except basic, reference, nominal, maximum, or minimum dimensions must have a tolerance. GDT is a
method of specifying the functional geometry of an object.
Understanding the international engineering language of GDT is essential for communicating in to-
day’s highly competitive global marketplace. You are required to read and interpret GDT symbols, which
provide detailed information about the function and relationship of the various part features.
70 Chapter 4
Figure 4.11 A table showing examples of nominal and actual sizes for framing lumber and boards.