How x-height and Ascender/Descender Length Impact Typeface Legibility

A common belief among type designers is that a larger x-height improves legibility, as the central part of letters, which occupies the x-height area, is considered critical for letter recognition. Additionally, perceived font size is often influenced more by the x-height than by the font’s point size.
However, increasing the x-height comes with potential trade-offs. If the x-height becomes too large, it reduces the lengths of ascenders and descenders—features that distinguish letters like "h" from "n" or "o" from "d." This raises important questions: Does an enlarged x-height enhance or hinder legibility at the letter and word level? Could it lead to new misreadings between visually similar letter pairs?
To explore these questions, we conducted a series of controlled experiments that examined the impact of x-height and ascender/descender length on letter and word recognition. The findings revealed that while increasing the x-height improves letter encoding efficiency, the effect is not uniform across all letters. Letters without ascenders or descenders benefit most. Both x-height and ascender/descender length were found to play independent yet essential roles in legibility, highlighting the need for balance between these design elements to optimize typeface clarity and readability.