Since the dawn of the printing press, designers and publishers have been concerned with font sizes. Even with the proliferation of electronic word processing, font size is still an important concept. The font size tells you how large the average character will be when it is printed on paper. It is easy to convert from font size points to inches, but the exact dimensions of a given character may vary drastically from font to font.
Conventions of Font Size
The point size of a font refers to the height of the font from the bottom of a descender of a letter to the top of an ascender of another letter. A descender is the part of a letter that hangs below the line, such as occurs in lowercase p and j. An ascender is the part that ascends up from the body of a letter, as seen in lowercase b and d. The font size refers to the total height spanned by the characters within a font. In most fonts, all the descenders are the same length, as are the ascenders.
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Basic Conversion of Font Size to Inches
An inch equals 72 points, so the basic conversion ratio for font size into inches is that 1 point equals 1/72 inch. For example, a 36-point font size is equal to 1/2 inch from the bottom of the descender to the top of the ascender. A single letter in the font will likely never achieve this height. A better way to think of the size is that the characters of the font, when typed adjacent to one another, exist in a space that is never more than 1/2 inch tall.
Another important parameter of font size measurement is the x-height. The x-height describes the height of the lowercase letter x. This is a way of expressing the height of a character without an ascender or descender. If the x-height of a font is large, many of the characters will appear larger than the characters in a font of the same size with a smaller x-height. For example, consider a 36-point font with an x-height of 24 points versus a 36-point font with an x-height of 12 points. The first font would have an x-height of 1/3 inch, whereas the second would have an x-height of 1/6 inch.
The width of a font size is difficult to express; the average width of characters within a font is far less standardized than the height. For example, within a font, characters such as an uppercase M span a large width whereas a lowercase i will be narrow. To describe the allowable width of a font, designers use the concept of the em box. The em box is the theoretical box containing a capital M in the font. This describes the maximum space that any one character can occupy. One em correlates directly to one point. For example, an em square in 36-point type would be 36 points high and 36 points wide. In inches, this is 1/2 inch high and 1/2 inch wide, meaning none of the characters would be wider than 1/2 inch.