While leading has existed since the days of metal type, Auto Leading is a relatively new concept, emerging with desktop publishing in the mid-1980s. Auto Leading allows InDesign to automatically assign a leading value to the text you set, based on the type's point size.
The advantage of Auto Leading is convenience. You can change your text size as many times as you like and your type will always be readable. As your font size increases or decreases, so does your leading. Although this can be useful when experimenting with type sizes, Auto Leading will probably cause more problems than it solves.
Don't by seduced by Auto Leading. Here's why:
Auto Leading is proportional to your type sizebut specific to the biggest piece of type in the paragraph. This means that if you have one word larger than the rest of the paragraph, you leading value will be 120 percent of the largest word or character. Leading values in parenthesis signify Auto Leading. By default, Auto Leading is 120 percent of your type size although you can change this in your Justification options.
Figure 4.9. Inconsistent leading caused by a single word in the paragraph set larger (example A); the leading converted to an absolute number (example B).
Figure 4.10. Changing the Auto Leading Amount.
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Auto Leading doesn't give you the kind of control of your text you need. Sure, if you're using 10-point type Auto Leading is 12 point, a nice easy number to work with. However, if you're working with 11-point type, then your leading value is 13.2, which is a cumbersome number, difficult to calculate in multiples if you intend working with a baseline or leading grid.
While Auto Leading works OK for body text, it can look terrible when applied to display type, which generally requires less leading. This is especially true if you have headings in all caps, because there will be no descenders to fill the lines.
Trust your eye, not your software, to determine how much leading you need. Auto Leading is useful when experimenting with type sizes, but when you decide upon the size you need be sure to convert your leading values to an absolute number, even if the Auto Leading value is the same as that number.
Figures 4.11A and 4.11B. Auto Leading applied to a headline (A). While Auto Leading works adequately for the body text, in the headline it is disproportionately large (B).
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Auto Leading is useful when using inline graphicspicture frames that have been pasted into a text frame, usually on their own line, and move with the flow of text. Inline graphics are a hybrid between text and graphics: You control the spacing of inline graphics using leading and Auto Leading ensures there's always enough space for the graphic on the line. An Auto Leading value of 100% works well for most inline graphics.