Citation?
The Elements of Typographic Style is quoted on the [url=“http://webtypography.net/Rhythm_and_Proportion/Vertical_Motion/2.2.2/”]Elements of Typographic Style Applied to the Web where 45-75 for single-column content, and 40-50 characters are for multi-column content.
http://webtypography.net/Rhythm_and_Proportion/Vertical_Motion/2.2.2/
Then comes the Agile Documentation book which in the Layout and Typography section has a section (page 101) explicitly about this:
[indent]Two Alphabets per Line
Problem - What is the optimum line width?
Forces - When reading, the reader’s eyes travel along the line from left to right. The eyes make small, jerky movements called ‘saccades’, between which there are periods called ‘fixations’. Fixations last for about a quarter of a second, while saccades are only 0.01 seconds long. It is during the fixations that information is picked up (Crowder 1982).
A line break interrupts the eye movement along the line. The reader’s eyes have to shift back to the beginning of the next line. Short lines increase the number of line breaks. If lines are too short, the reader’s eyes have to find the beginning of the next line more often than necessary, which breaks the flow of reading and makes reading tiresome (Conover 1985, Gulbins Kahrmann 1992).
On the other hand, lines that are too long also make reading difficult and tiresome. Long lines make it difficult for the reader’s eyes to follow a line and to find the beginning of the next line once a line break occurs (Conover 1985, Gulbins Kahrmann 1992).
Moreover, the optimum line width depends on the typeface and type size used. Type set in larger sizes requires longer line widths (Conover 1985, Gulbins Kahrmann 1992).
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This is followed by a Solution section that specifies a lower limit of one-and-a-half alphabets, and an upper limit of two-and-a-half alphabets per line, along with a Discussion section that goes in to how page layout, justification, and line spacing can affect this in different subtle ways.