Characters per line
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In typography and computing characters per line (CPL) or terminal width refers to the maximal number of monospaced characters that may appear on a single line. It is similar to line length in typesetting.
At the end of the typewriter age, most designs were geared toward 72 CPL, derived from a pitch of 12 characters per inch, multiplied by 6 inches (see for example IBM Selectric). This would ensure at least 1 inch for each margin, with the U.S. government at the time having standardized on 8 1/2×11" paper. Early computer text editors used this measure as a baseline for their terminal interfaces. Other common measures were 80 CPL (historically descended from punched cards and later broadly used in monitor text mode), 40 CPL (in low-resolution screens) and 132 CPL.
In modern computing 
RFC 2046 Media Types November 1996 A. Collected Grammar .................................... 43 1. Introduction The first document in this set, RFC 2045, defines a number of header fields, including Content-Type. The Content-Type field is used to specify the nature of the data in the body of a MIME entity, by giving media type and subtype identifiers, and by providing auxiliary information that may be required for certain media types. After the
With the advent of desktop computing and publishing, and technologies such as TrueType used in word processing and web browsing, a uniform CPL has been made mostly obsolete. HTML (and some other modern text presentation formats) uses dynamic word wrapping which is more flexible than characters per line restriction and may produce a text block with non-rectangular shape, just like in paper typesetting.
Many plain text documents still conform to 72 CPL out of tradition.
Human perception 
A study at Wichita State University found that CPL had only small effects on readability, including factors of speed and comprehension. When asked for preferences, however, 60% of respondents indicated a preference for either the shortest (35 CPL) or longest (95 CPL) lines used in the study. At the same time, 100% of respondents selected either one of these quantities as being the least desirable.
See also 
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