The font size of subheadings doesn’t have to be larger than the font size of body text. It’s sufficient to make the text bold and to add some extra space before the paragraph.
If you decide to use a larger font size, don’t make it more than 2 points larger than body text.
Always add more space before the subheading than after the subheading so that it’s clearly visible that the following text belongs to the subheading. A good value for the distance between the preceding text and the subheading is about one blank line. The space after the subheading should be about one-third of this.
Clearly distinguish subheadings from major topic headings. One good way of doing so is to color subheadings gray. This makes them unobtrusive even if they’re bold.

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