The required sizes of structural features are conveyed through use of dimensions. Distances may be indicated with either of two standardized forms of dimension: linear and angular. A third type of dimension that is commonly used for curved structures are diametrical or radial dimensions.
With linear dimensions, there are two parallel lines called extension lines spaced at the distance between two features shown at each of the features. A dimension line, which is perpendicular to the extension lines and has arrows at its endpoints, is shown between them, terminating at the extension lines. The distance is indicated numerically, either adjacent or in a gap provided, at the midpoint of the dimension line. Note the dimension shown in Figure 7.
With angular dimensions, the extension lines establish an angular relationship between a feature and a reference line or another feature. Note the 22 degree angle shown in Figure 8.
Sizes of circular structural features are indicated using either diametrical or radial dimensions. Radial dimensions use an R followed by the value for the radius; diametrical dimensions use a circle with a forward-leaning diagonal line through it, called the diameter symbol or D, followed by the value for the diameter. A radially aligned line with arrowhead pointing to the circular feature, called a leader, is used in conjunction with both diametrical and radial dimensions. An example of a radial dimension is shown in Figure 9.