Which part is NOT considered the x-height?

The x-height of a typeface refers specifically to the height of the lowercase letters, particularly measured from the baseline to the mean line (often represented by the height of the letter 'x'). This measurement is an essential aspect of typography as it influences the readability and overall appearance of text.

The top of the "I" is not considered part of the x-height because the x-height is defined by the height of lowercase letters, while the "I" is an uppercase letter. Its top extends beyond the x-height, making it irrelevant when discussing the specifics of x-height.

In contrast, the bottom of a lowercase letter and the height of the lowercase 'x' are integral components of defining the x-height, as they both pertain to the measurement of lowercase letterforms. Additionally, the line on which text sits, known as the baseline, provides the foundational reference for measuring the x-height but is not a part of the x-height itself. Thus, identifying the top of the "I" as unrelated to the x-height underlines the distinction between the elements that define it and other typographic features.