54 Chapter 4
of dimensions are typically composed of two parts: the nominal value, which is the “ideal” size of the fea-
ture, and the tolerance, which specifies the amount that the value may vary above and below the nominal.
The Metric System.
The metric system originated in France in the 1790s as an alternative to the traditional English units of
measurement. It was intended to standardize the units of measurement to assist in the expansion of
trade and commerce throughout continental Europe. Today the majority of countries around the world
have adopted an exclusively metric system of measurement. The United States, however, remains one
of the few countries that have not adopted this system. It is expected that the construction industry will
eventually convert to the metric system. Some of its many advantages include the elimination of frac-
tions on drawings, simpler calculations, and international uniformity.
Dimension Conventions.
There are a number of conventions that relate to dimensioning that the drafter and blueprint reader
should be aware of:
Figure 4.1 A diagram showing the different types of dimensions.