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An obelus (symbol: ÷ or †, plural: obeluses or obeli) is a symbol consisting of a short horizontal line with a dot above and below, and in other uses it is a symbol resembling a small dagger. In mathematics it is mainly used to represent the mathematical operation of division. It is therefore commonly called the division sign. Division may also be indicated by a horizontal line (fraction bar) or a slash. In ISO 80000-2-9.6 (about division) it says verbatim "The symbol ÷ should not be used." In editing texts an obelus takes the form of a dagger mark (†) and is used as a reference mark, or to indicate that a person is deceased, and often used to indicate a footnote.
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In mathematics, the obelus symbol has also been used to represent subtraction in Northern Europe.
History
The word "obelus" comes from ὀβελός, the Ancient Greek word for a sharpened stick, spit, or pointed pillar. This is the same root as that of the word "obelisk". Originally this sign (or a plain line) was used in ancient manuscripts to mark passages that were suspected of being corrupted or spurious; the practice of adding such marginal notes became known as obelism. The dagger symbol, also called an obelisk, is derived from the obelus and continues to be used for this purpose. As Streeter (1924) writes:
The obelus, invented by Aristarchus to mark suspected passages in Homer, is frequent in manuscripts of the Gospel to mark just those sections, like the Pericope in John, which modern editors reject. The first corrector of [the Codex Sinaiticus], probably the contemporary διορθωτής, was at pains to enclose in brackets and mark with dots for deletion two famous passages in Luke written by the original scribe which, being absent from B W 579 and the Egyptian versions, we infer were not accepted in the text at that time dominant in Alexandria, viz. the incident of the "Bloody Sweat" in Gethsemane (Lk.xxi.43 f.) and the saying "Father forgive them" (Lk.xi.34).
Although previously used for subtraction, the obelus was first used as a symbol for division in 1659 in the algebra book Teutsche Algebra by Johann Rahn. Some think that John Pell, who edited the book, may have been responsible for this use of the symbol. The usage of the obelus to represent subtraction continued in some parts of Europe (including Norway and, until fairly recently, Denmark). Other symbols for division include the slash or solidus (/), and the fraction bar (the horizontal bar in a vertical fraction).
In computer systems
In Microsoft Windows, the obelus is produced with Alt+0247 on the number pad or by pressing Alt Gr+⇧ Shift++ when an appropriate keyboard layout is in use. In classic Mac OS and macOS, it is produced with ⌥ Option+/.
On UNIX-based systems using Screen or X with a Compose key enabled, it can be produced by composing : (colon) and - (hyphen/minus), though this is locale- and setting-dependent. It may also be input by Unicode code-point on GTK-based applications by pressing Control+⇧ Shift+U, followed by the codepoint in hexadecimal (F7) and terminated by return.
In the Unicode character set, the obelus is known as the "division sign" and has the code point U+00F7. In HTML, it can be encoded as ÷ or ÷ (at HTML level 3.2), or as ÷.
In LaTeX, the obelus is obtained by \div.