This is “Combining Fonts and Effects”, section 1.2 (from appendix 1) from the book Designing Business Information Systems: Apps, Websites, and More (v. 1.0). For details on it (including licensing), click here.
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Normally, this is done to create contrast and visual interest.
The rule for combining fonts is very simple—you may combine fonts in a document as long as each font comes from a different category. For example any of the combinations on the opposing page will work.
Want even more drama? Contrasting techniques may also be used in combination for dramatic effect. See examples on opposite page.
Examples of Combining Fonts
Examples of Combining Contrasting Techniques