Choose a comfortable measure

Anything from 45 to 75 characters is widely regarded as a satisfactorily length of line for a single-column page set in a serifed text face in a text size. The 66-character line (counting both letters and space) is widely regarded as ideal. For multiple-column work, a better average is 40 to 50 characters.

If the type is well set and printed, lines of 85 or 90 characters will pose no problem in discontinuous texts, such as bibliographies, or, with generous leading, in footnotes. But even with generous leading, a line that averages more than 75 or 80 characters is likely to be too long for continuous reading.

A reasonable working minimum for justified text in English is the 40-character line. Shorter lines may compose perfectly well with sufficient luck and patience, but in the long run, justified lines averaging less than 38 or 40 characters will lead to white acne or pig bristles:

When the line is short, the text should be set ragged right. In large doses, even ragged-right composition may look anorexic if the line falls below 30 characters, but in small and isolated patches – ragged marginal notes, for example – the minimum line (if the language is English) can be as little as 12 or 15 characters.

These line lengths are in every case averages, and they include empty spaces and punctuation as well as letters. The simplest way of computing them is with a copy-fitting table. Measure the length of the basic lowercase alphabet – abcdefghijklmnopqrstuvwxyz – in any face and size you are considering, and the table will tell you the average number of characters to expect on a given line. In most text faces, the 10 pt roman alphabet will run between 120 and 140 points in length, but a 10 pt italic alphabet might be 100 points long or even less, while a 10 pt bold might run to 160. The 12 pt alphabet is, of course, about 1.2 times the length of the 10 pt alphabet — but not exactly so unless it is generated from the same master design and the letter-fit is unchanged.

On a conventional book page, the measure, or length or line, is usually around 30 times the size of the type, but lines as little as 20 or as much as 40 times the type size fall within the expectable range. If, for example, the type size is 10 pt, the measure might be around 30 x 10 = 300 pt, which is 300/12 = 25 picas. A typical lowercase alphabet length for a 10 pt text font is 128 pt, and the copy-fitting table tells us that such a font set to a 25-pica measure will yield roughly 65 characters per line.