Linear dimensions, or linear units, are used to measure the distance between two points. Since two points define a line, the units of distance are sometimes called "linear" units or dimensions. In the metric system linear dimensions are generally in millimeters. To avoid having to specify "mm" after every dimension, a label such as "all dimensions in mm" or "unless otherwise stated all dimensions are in mm" is usually contained in the title block. If the dimension is less than one, a leading zero should be used before the decimal point: e.g., 0.5. Linear units can also be in centimeters and inches, meters and feet, or kilometers and miles, to name a few.

In architectural and engineering drawing, the most important dimensions determine subsequent dimensions, providing a dimension standard. Thus, if a wall is dimensioned to its center first, all following dimensions using this wall as a reference point should be dimensioned at the center.

Projection lines are used to indicate the extremities of a dimension. Dimension lines are used to label a particular dimension. Thin lines are used for both projection lines and dimension lines (Figure 4.7).

Area dimensions are two-dimensional and measure area. They are often but not always expressed as squares of linear dimensions: square inches or inches squared (in2), square feet or feet squared (ft2), and square meters or meters squared (m2). A rectangle that is 8 feet long by 4 feet wide, for example, has an area of 32 square feet (8 linear feet times 4 linear feet). Examples of units of area that are not a square of a linear unit are the acre and hectare. There are others.

Volume dimensions are three-dimensional and are expressed as the cube of linear units. A cube that measures 2 feet on each edge has a volume of 2 x 2 x 2 = 8 cubic feet.

Two methods of dimensioning are in common use:

• Unidirectional: The dimensions are written horizontally.

• Aligned: The dimensions are written parallel to their dimension lines. Aligned dimensions should always be readable from the bottom or the right of the drawing.

When several dimensions make up an overall length, the overall dimension may be shown outside the component dimensions. When specifying an overall dimension, one or more noncritical component dimensions must be omitted (Figure 4.8A).

When all of the component dimensions must be specified, an overall length may still be specified as an auxiliary dimension. Auxiliary dimensions are never toleranced and are shown in brackets (Figure 4.8B).

Dimensions that are not drawn to scale are underlined (Figure 4.9A). When a dimension line cannot be completely drawn to its normal termination point, the free end is terminated in a double arrowhead (Figure 4.9B).

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B. Auxiliary Dimension

Figure 4.8 Two line drawings depicting the use of overall dimension (A) and auxiliary dimension (B).

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