Type size is measured from the top of the tallest letter to the bottom of the lowest descender. Actually, the measured height of type is just slightly higher and lower than those limits. The first types were made by pouring molten lead into molds, which were slightly tapering so the lead could be released after it cooled. The bottom of the cast letter—and the sides of the type itself—was bigger than the printing surface. Type was measured by the size of the body, not the surface.
(BTW, this isn’t an error. Physical type is oriented this way by typesetters and printers. It’s easier to read upside-down, mirror-image type because of our deeply ingrained, life-long habit of reading from left to right. The face is Baskerville, identified by its characteristic g, whose bottom loop doesn’t close, and the cleat on the numeral 2.)
Printers, typographers, and book and graphic designers refer to the “x-height’’ of letters, meaning the height of the lowercase x and all other lowercase letters that do not have ascenders (parts that stick up, like a d and b) or descenders (parts that hang down, like a g and y). Some typefaces have long ascenders and descenders, and thus a small x-height. Bernhard Modern is such a face. Some have short ascenders and descenders, and a high x-height. Antique Olive is like that.
The result of this is that the smaller the x-height, the more apparent space between lines and the more characters that can be fit in a given length. You can fit more text in the same space. Text set in a font with a high x-height has less apparent space between lines and fewer characters in a given length. You can’t fit as much text in the space.
This illustrates how the x-height of a font can affect the appearance of the text:
