Whew! The hard part is getting it to look right. After you've copied or created a suitable dimension style, you're ready to dimension. Fortunately, adding dimensions to a drawing with existing dimension styles is usually pretty straightforward.
When you want to dimension something in AutoCAD, you can either select the object, such as a line or polyline segment, or select points on that object, such as the endpoints of the line or polyline segment. If you select an object, AutoCAD finds the most obvious points on it to dimension, such as the endpoints of a line. If you choose to select individual points instead, use object snaps (see Chapter 7). When you change the size of the object (for example, by stretching it), AutoCAD automatically moves the dimension's origin points and updates the dimension text to show the new length.
If you don't use object snaps or another AutoCAD precision technique to choose dimension points, the dimension text probably won't reflect the precise measurement of the object. This lack of precision can cause serious problems. When in doubt, OSNAP to it!
When you set up a new drawing and you want to use annotative dimensions, make sure the Annotative box is selected on the Fit tab in the New/Modify Dimension Style dialog box (refer to Figure 14-7). For a new drawing with non-annotative dimensions, change the Use Overall Scale Of setting ...