Characters per line

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.

History

The ruler on the carriage of an Olivetti Lettera 22. This typewriter can print only 87 characters in a line

The limit of the line length in 70–80 characters may well have originated from various technical limitations of various equipment. The American teletypewriters could type only 72 CPL, while the British ones even less, 70 CPL.[1] In the era of typewriters, most designs of the typewriter carriage were limited to 80–90 CPL. The most widespread and standard paper size in the US (8.5×11") also has been imposing limitations on the line length: it is only possible to print a maximum of 85 or 102 characters (with the font size either 10 or 12 characters per inch) without margins on the typewriter. With various margins (usually from 1 to 1.5 inches for each side, but there is no strict standard) these numbers may shrink to 55–78 CPL.

Typometer with the characters per line scales

A Fortran coding form (paper). Source code has 72 CPL, but a form is 80-characters wide. Last 8 positions are "identification sequence"

In computer technology, a line of an IBM punched card could consist of only 80 characters. The widespread computer terminals such as IBM 3270 followed this limitation, their monitors could show only 80 CPL (but with the various number of lines), though with some terminals this number was either reduced by half to 40 CPL, limited to 64 CPL (SWTP CT-64, with 16 lines[2]), or optionally increased to 132 CPL (DEC VT100 family, with 14 lines[3]). Such line lengths have been carried over into text modes of personal computers.

The "long" line of 132 CPL comes from line printers of mainframes.[4][5][6] However, some printers or printing terminals could print as many as 216 CPL, given certain extra-wide paper sizes and/or extra-narrow font sizes.[7]

In modern computing

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 (e.g., RFC 678).

In programming

Many style guides for computer programming define the maximum or desirable number of characters in a line of source code:

Characters per line	Programming style
72	Ada[8]
79	Python[9][10]
80	GNU[11] Google C++[12] Objective-C/C++[13] Python[14] R[15] JavaScript[16] Java[17] Mozilla[18] Object Pascal[19] Perl[20] PHP[21] Ruby[22]
90	CCM4[23]
100	Android[24] Google Java[25] Common Lisp[26][27] Linux kernel[28]
120	PHP[21]
132	Fortran[29] Blink[30] Moodle[31]
180	Mono[32]
undefined	Go[33] JavaScript (JavaScript has no official style guide)

See also

• IBM 80-column punched card format
• Column (typography)
• Line length (the equivalent concept for non-monospaced text)

References

1. Department of the Army, ed. (1947). Teletypewriter Circuits and Equipment (fundamentals). Washington: US Government Printing Office. p. 69.
2. CT-64
3. VT100
4. Pomerantz, Ori; Vander Weele, Barbara; Nelson, Mark; et al., eds. (2008). Mainframe Basics for Security Professionals.
5. Wells, April J. (2003). Oracle 11i E-Business Suite from the Front Lines. p. 168.
6. "Difference between..LRECL = 133 and LRECL = 132". IBMMAINFRAMES.com - IBM Mainframe Support Forums. 2004.
7. "Appendix K. Traditional Terminals and Printers". Terminals & Printers Handbook 1983–84. Digital. 1983.
8. Ada 95 Quality and Style Guide
9. PEP 8 Style Guide for Python Code
10. Style Guide for Python Code
11. GCC Coding Conventions
12. Google C++ Style Guide
13. Chromium Objective-C and Objective-C++ style guide
14. Google Python Style Guide
15. Google's R Style Guide
16. Google JavaScript Style Guide
17. "4.1. Line length". Java Code Conventions (PDF). Sun Microsystems, Inc. 1997. p. 5.
18. Mozilla Coding style
19. Object Pascal Style Guide
20. Conway, Damian (2005). Perl Best Practices: Standards and Styles for Developing Maintainable Code. O'Reilly. p. 40. ISBN 978-0-596-55502-3.
21. PSR-2: Coding Style Guide
22. The Ruby Style Guide
23. CCM4 self-imposed limit
24. Android Code Style Guidelines for Contributors
25. Google Java Style
26. Common Lisp Style Guide
27. Google Common Lisp Style Guide
28. "Linux kernel code style as of June 2020". git.kernel.org. Retrieved 2020-06-13.
29. FORTRAN 90
30. Blink Coding Style Guidelines
31. Moodle Coding Style
32. Mono Coding Guidelines
33. Effective Go