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A question mark is a punctuation mark. It usually marks a full stop, replacing the period at the end of an interrogatory sentence, such as "Where did you get that hat?". It can also be used mid-sentence to mark a merely interrogative phrase, where it functions similarly to a comma, as: "Where shall we go? and what shall we do?" but this use is increasingly rare.
For use of spaces after a question mark, see the discussion for the full stop.
The symbol is generally thought to originate from the Latin quæstio, meaning question, which was abbreviated to 'Qo'. The capital 'Q' was written above the lowercase 'o', and this mark was transformed into the modern symbol.
Another theory about the origin of the question mark proposes that the mark originated in the 9th century, when it appeared as a point followed by the curvy bit written slanted (like a tilde, although the tilde was tilted more upward to the right). The point has always indicated the end of a sentence. The curved line represented the intonation pattern of a spoken question, and may be associated with a kind of early musical notation, like neumes.
In some languages, most notably in Spanish typography since the 18th century, every question mark must be opened and closed; an interrogative sentence or phrase begins with an inverted question mark, "¿" and ends with the familiar question mark "?". However, this orthographical tradition is often disregarded in quick typing, especially in chat rooms and Internet forums, and where the inverted character is not easily available from the keyboard.
In Greek a semicolon ";" is used as a question mark.
In Arabic the question mark "؟" is inverted, but not upside down as in Spanish but mirrored, and only used at the end of the sentence, unlike Spanish. In Hebrew the ordinary (European) question mark is used, as in English.
The rhetorical question mark first appeared in the 1580s and was used at the end of a rhetorical question. It was the reverse of an ordinary question mark, so that instead of the main opening pointing back into the sentence, it opened away from it. This use disappeared in the 1600s.
In computing, the question mark is represented as Unicode and ASCII character 63 or 0x003F. The inverted question mark corresponds to Unicode character 191 (0x00BF), and can be accessed from the keyboard in some versions (depends on ANSI codepage) of Microsoft Windows by holding the [Alt] key and typing 0 1 9 1 on the numeric keypad. In GNOME applications, it can be entered by typing the hexadecimal Unicode character while holding ctrl-shift, i.e.: ctrl-shift BF - ¿. In recent XFree86 and X.Org incarnations of the X Window System, it can be accessed as a compose sequence of two straight question marks, i.e. pressing <Compose> ? ? yields ¿.
The question mark is used in ASCII renderings of the International Phonetic Alphabet, such as SAMPA in place of the glottal stop symbol (which resembles "?" without the dot), and corresponds to Unicode character U+0294 Latin letter glottal stop (ʔ).
In computer programming, the symbol "?" appears in several programming languages. In C it is part of the ?: operator, which is used for simple boolean conditions. In the POSIX syntax for regular expressions, such as the one used in Perl and Python, ? stands for "zero or one instance of the previous subexpression", i. e. an optional element.
- Ellen Lupton and J. Abbott Miller, Period Styles: A Punctuated History
- M. B. Parkes, Pause and effect: an introduction to the history of punctuation in the West
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