Use consistent style on chapter, header titles
When writing chapter titles and section headers in nonfiction books, you’ll want to use a consistent style. For example, if one section title is written as What Hiking Apparel to Buy for Children, but the next one is written as Where to buy hiking apparel, the styles are inconsistent as the former is capitalized but the latter is lowercased.
Few readers may notice that you're using inconsistent styles, but typographically the manuscript will feel messy to the reader...and the jump from messy appearance to messy content is not a difficult leap for most readers.
Generally, with chapter titles and section headers, you can use one of two styles: up style or down style.
Up style consists of capitalizing the first and last words in the title or headline and all “major words” in the title. An example is Five Ways to Boost Book Sales. Its benefit is that it tends to stand out more than down style, so if using a font size that's the same as or close to the text, the header won’t get lost. The disadvantage is that there’s disagreement in style guides over what is a major word. Usually articles (a, an, the), prepositions, and coordinating conjunctions (and, but, or, for, nor) are lowercase. While the Chicago Manual of Style says prepositions should be lowercase unless the first word, the Associated Press stylebook says to capitalize them. To wit, the above example in this paragraph is written using CMOS while AP would capitalize to.
Down style requires that only the first word of the title and any proper nouns be capitalized. An example is Five ways to boost Kindle ebook sales. It’s benefit is that it offers easier rules to follow than with up style, and there’s no argument over which is the first word or a proper noun. The major disadvantage is that it tends to get lost if the font size is the same as or similar to the text's font size. Of course, this disadvantage can be overcome by centering the header, increasing its font size to at least 6-12 points greater than the text, or by giving the header a different color than the text.
A final note: Either style is fine to use. Just choose one and stick with it.
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