In typography, the invisible ceiling to which (most) capital letters in a line of type rise is called the capline. The height of the capline, and the cap height of a font is defined as the height of the X.

Capital, or majuscule letters with flat tops (most often BDEFHIJKLPRTUVWXYZ) meet the capline. What may not be so obvious is that font designers take care to try and preserve the capline optically rather than mathematically. To that end, the tops of letters with bowls (CGOQS) and, if the A is designed with a point, that too, actually rise slightly above the capline.

Whether or not the lower-case (minuscule) letters with ascenders (bfkl) meet the capline depends on the font.

The uneven tops of words are their most distinct bouma feature, and writing in all capitals overemphasizes the capline, which is why ALL CAPS WRITING IS A LITTLE MORE DIFFICULT AND SLOW though not prohibitively so. The capline of a font is reinforced by serifs, and bracketed serifs even moreso.

See also baseline, x-height