Arrow (symbol)
Graphical symbol or pictogram used to point or indicate direction
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History
An older (medieval) convention is the manicule (pointing hand, ☚). Pedro Reinel in c. 1505 first used the fleur-de-lis as indicating north in a compass rose; the convention of marking the eastern direction with a cross is older (medieval).
Use of the arrow symbol does not appear to pre-date the 18th century. An early arrow symbol is found in an illustration of Bernard Forest de Bélidor's treatise L'architecture hydraulique, printed in France in 1737. The arrow is here used to illustrate the direction of the flow of water and of the water wheel's rotation. At about the same time, arrow symbols were used to indicate the flow of rivers in maps.
A trend toward abstraction, in which the arrow's fletching is removed, can be observed in the mid-to-late 19th century. The arrow can be seen in the work of Paul Klee. In a further refinement of the symbol, John Richard Green's A Short History of the English People of 1874 contained maps by cartographer Emil Reich, which indicated army movements by curved lines, with solid triangular arrowheads placed intermittently along the lines.
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Main usage
Arrows are universally recognised for indicating directions. They are widely used on signage and for wayfinding, and are often used in road surface markings. A two-way road may be indicated by "↕" or "⇅".
In mathematics and sciences
Upward arrows are often used to indicate an increase in a numerical value, and downward arrows indicate a decrease.
In mathematical logic, a right-facing arrow indicates material conditional, and a left-right (bidirectional) arrow indicates if and only if, an upwards arrow indicates the NAND operator (negation of conjunction), an downwards arrow indicates the NOR operator (negation of disjunction).
Use of arrow symbols in mathematical notation developed in the first half of the 20th century. David Hilbert in 1922 introduced the arrow symbol representing logical implication. The double-headed arrow representing logical equivalence was introduced by Albrecht Becker in Die Aristotelische Theorie der Möglichkeitsschlüsse, Berlin, 1933.
Knuth's up-arrow notation uses multiple up arrows, such as ⇈, for iterated, or repeated, exponentiation (tetration).
The quantum theory of electron spin uses either upward or downward arrows.
A vector may be denoted with an overhead arrow, such as in or .
Some computer programming languages, like Haskell, use arrows (typed as ->) to denote a type class.
In chemistry, arrows indicate a chemical reaction. For example, 2H2 + O2 → 2H2O shows that hydrogen and oxygen react to produce water.
Graffiti
Arrows are regularly used in contemporary graffiti designs, incorporated as an element in both simplistic tags and complex wildstyle pieces. Arrows are used as a visual technique to make a graffito stand out or give it a sense of movement. The graffiti theoretician RAMM:ΣLL:ZΣΣ described adornments, such as arrows, in wildstyle paintings as ornaments that ‘armed’ the letters of a piece. The Philadelphia graffiti artist Cool Earl began using arrows in 1967, although the New York graffiti writer SJK 171 may have been the first to do so. The graffiti artist Mare139 is known for creating 3D sculptures of arrows.
Unicode
In Unicode, the block Arrows occupies the hexadecimal range U+2190–U+21FF, as described below.
By block
Additional arrows can be found in the Combining Diacritical Marks, Combining Diacritical Marks Extended, Combining Diacritical Marks for Symbols, Halfwidth and Fullwidth Forms, Miscellaneous Mathematical Symbols-B, Miscellaneous Symbols and Pictographs, Miscellaneous Technical, Modifier Tone Letters and Spacing Modifier Letters Unicode blocks.