Typography defines the x-height as the height of a lowercase letter ‘x’ that does not extend above or below its mean line, measuring overall quality and legibility at small sizes.
The x-height of a typeface can also indicate its personality; typefaces with larger x-heights tend to appear friendlier and approachable, while those with smaller ones appear more formal or serious.
The x-height of a font is typically expressed as a percentage of its cap height (i.e., the height of capital letters). A font with an x-height of 70% would feature capital letters with heights equal to that percentage – thus providing its name.
The x-height is an essential element in the legibility of a font. A font with a larger x-height will generally be easier for readers at smaller sizes to read than ones with smaller ones; as its primary purpose is forming lowercase letters visible against a background, having a larger x-height makes distinguishing one letter from another much simpler.
The x-height of a font refers to the height of its letters in lowercase, which plays an integral part in its readability. Fonts with larger x-heights are more manageable for readers than ones with small ones, with the former offering greater spacing between letters than its smaller counterpart. This factor alone can make a dramatic difference in readability when used in smaller sizes.