X-height is a typeface property: the height of its lowercase letters measured upward from the baseline. It is not the size you assign to text; two fonts set at the same apparent scale can still have different x-heights. section 5.1.7
Figure 5.7 A demonstration of the different x-heights (baseline to red lines) for five fonts, sized so they all have the same cap height, which is the distance from the baseline to the top of the uppercase letters (marked by the top blue line)
Compare the red x-height line with the shared cap-height line: matching uppercase height does not make lowercase letters equally tall. section 5.1.7
For the small labels, annotations, captions, and occasional text blocks common in data visualizations, x-height is part of the readability test. Favor typefaces that leave a comfortable balance of ink and white space; large x-heights and well-rounded counters tend to support readability. Do not judge x-height alone, though. Font weight, counter shape, and whether easily confused characters such as lowercase l, 1, and uppercase I remain distinct also matter. section 5.2.1 section 5.6
Same cap height, different lowercase presence
Set several candidate fonts to the same cap height, then inspect a short passage at its intended small size. The lowercase text may feel more or less legible even though the uppercase letters align. Zooming in can reveal the letterform details behind that result; zooming back out checks whether they survive ordinary use. section 5.1.7 section 5.2.1 section 5.3
X-height also contributes to tone. A smaller x-height appears among the details that give Goudy Old Style a daintier, warmer, more diminutive feeling than the solid, modern feel described for Roboto. Readability and personality are therefore related design considerations, not a single slider with a magic setting. section 5.3
