In the EBCDIC character set formerly used on IBM mainframe computers and their clones there were two vertical bar symbols: | and ¦. The first was intended to be used to mean “or” in computer languages like PL/I (and to be used for any other meaning a vertical bar might be used for in text or mathematics). It now appears in Unicode as VERTICAL LINE ( http://unicode.org/cldr/utility/character.jsp?a=|&B1=Show ). The second was to be used to mark the beginning or end of a column in print-outs.
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¦ xxxxxxxx ¦ xxxxxxx ¦
¦ xxxxxxxx ¦ xxxxxxx ¦
¦ xxxxxxxx ¦ xxxxxxx ¦
¦ xxxxxxxx ¦ xxxxxxx ¦
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It now appears in Unicode as BROKEN BAR ( http://unicode.org/cldr/utility/character.jsp?a=%C2%A6&B1=Show ).
The idea was that the hyphen (or the equal sign) could be used to build horizontal lines; and since the space allocated in a printout for a character in a Latin-letter fixed-width font was close to being twice as tall as it was wide, the corresponding sign for a vertical line was devised to appear as two vertical hyphens.
This was the first use of the character ¦ which had not been needed on typewriters were one would have just drawn any necessary table vertical separation lines.
However both | and ¦ were normally omitted from IBM chain printers and so were seldom seen.
When the second published version of ASCII was devised, the original intent was to include | as character 124 and not include ¦ at all, as | could also serve as a column separator. But some influential programmers complained that character 124 was in a national use position which meant that it was allowed to be replaced by a different character in non-English and non-US versions of ASCII. If this occurred, the programming symbol would be lost. So the ASCII committee decided to allow ! to also be allowed to look like | and mean “or”, and made the representation glyph for character 124 to be the broken bar so it would be obviously unsuited for “or”. See http://www.transbay.net/~enf/ascii/ascii.pdf .
However, the creators of the programming language c still used character 124 to mean “or” and used “!” to mean “not”, and eventually in ASCII character 124 was changed back to |.
The original MS-DOS code page 437 and some other MS-DOS code pages still show it as ¦ and on many keyboards it still incorrectly appears as ¦. Under Windows you can type ¦, if you are using the Window western European character set normal for English, by making sure Num Lock is on, pressing down the left Alt key, typing 0166 on the numeric keypad. then releasing the left Alt key. (The right Alt key also works on many systems.)
If you have a laptop, you must emulate the numeric keypad. See http://en.wikipedia.org/wiki/Fn_key for the most normal method.
For various uses of |, see http://en.wikipedia.org/wiki/Vertical_bar . The “pipe” usage is common in programming operating systems.
Unicode from the beginning has included two other similar symbols to be used in place of ASCII 124 in careful typography. One of these is the mathematical symbol ∣ ( http://unicode.org/cldr/utility/character.jsp?a=%E2%88%A3&B1=Show ) called DIVIDES which can also be used for any other vertical line symbol in mathematical or logical notation. In italic fonts where mathematical symbols remain upright, it should also remain upright. The other symbol is the letter ǀ ( http://unicode.org/cldr/utility/character.jsp?a=%C7%80&B1=Show ) called LATIN LETTER DENTAL CLICK which is used for the Khoisan dental click sound, formerly indicated by [ʇ] in IPA and written “c” in Zulu orthography. It should be slanted in an italic font like other Latin letters.
The symbol ¦ is not a letter in any conventional orthography and should never be slanted, even in an italic font, since its primary meaning remains “start or end of column”. It and any duplicates on the line above and the line below should appear to line up in a vertical line when the broken bar is being used as intended (which in contemporary use is very, very seldom).