X-height is based on the height of a font's lowercase letters

Capital Letters On White Background
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In typography, x-height is the distance between the baseline of a line of type and tops of the main body of lowercase letters, excluding ascenders, which are the strokes that go upward, or descenders, which are the tails or other parts of a letter that reach beneath the baseline. The x-height is a factor in typeface identification and readability. The letter X does not have any ascenders or descenders, which is why it was chosen for the comparison between fonts.

(Lowercase u, v, w, and z could have filled this position as well because they are boxy letters that don't have ascenders or descenders.) Curved letters tend to exceed the x-height a bit.

X-height is a relative typeface measurement that describes only general proportions of a typeface because the Xs in every font is not the same height. Xs set in the same size in multiples fonts will appear to be from different font sizes. Fonts with a large x-height look much bigger than a font with a small x-height of the same size. 

Point size is a measurement that takes all ascenders and descenders into consideration. X-height allows comparison of fonts based on the size of their lowercase letters.

Typefaces with a large x-height relative to the ascenders and height of the uppercase letters of the font have shorter ascenders and descenders and thus less white space between lines of type. Sans serif typefaces typically have large x-heights.

Considerations When Evaluating a Font's X-Height

  • Display faces with large lower case characters attract attention. Typefaces with large x-heights make any typeface more visible at any size.
  • Fonts with small x-heights are usually harder to read and should be avoided at small sizes.
  • Fonts with large x-heights take up more space that fonts with small x-heights. This could lead to a book being 10 percent longer in one font than in another, which affects printing and distribution costs.
  • In typefaces with small x-heights, other letter parts such as ascenders and descenders may become more visually noticeable.
  • Typefaces with large x-heights perform well as display or headline fonts but may appear darker, heavier, crowded or more difficult to read at body copy sizes.
  • If readability is a problem and changing to a typeface with a smaller x-height is not an option, you can open up the lines of type by adding more leading (line spacing) and not using fully justified alignment.
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Bear, Jacci Howard. "X-Height." ThoughtCo, Aug. 14, 2017, thoughtco.com/x-height-in-typography-1078327. Bear, Jacci Howard. (2017, August 14). X-Height. Retrieved from https://www.thoughtco.com/x-height-in-typography-1078327 Bear, Jacci Howard. "X-Height." ThoughtCo. https://www.thoughtco.com/x-height-in-typography-1078327 (accessed November 25, 2017).