What ↦ means
It gives an input the output its rule produces.
In x ↦ x², read “x maps to x squared”; this function rule gives the output for that input.
The shift is visible in Bourbaki, a collective of mathematicians, between 1963 and 1965: → gave way to ↦ for this rule. The bar separated an input’s output from the domain-to-codomain arrow.
What it means when someone sends it to you
2 readingsX ↦ x²
The rule sends x to its square.
Maplet arrow in Z notation: x ↦ y
The ordered pair has x first and y second.
Don't confuse it with
1Combination ideas
1Two inputs share the output 3.
Questions people ask
1Can ↦ name an unnamed function?
Yes. A rule such as x ↦ x² can define the function without a separate name.
How to type ↦
Windows (Word)
Alt+XType 21A6, then Alt+X to get ↦.
Windows
Win+Rrun charmap for the full table, or use the emoji picker's Ω symbols tab (Win + .).
macOS
Control+Command+SpaceOpen the character viewer and search “rightwards arrow from bar”.
Linux
Ctrl+Shift+UType 21a6, then Enter or Space.
HTML
↦CSS
\21A6Codes and encoding1 code points
HTML decimal
↦
HTML hex
↦
CSS content
content: "\21A6";
Each escape is closed by the next backslash; a trailing space is needed before a literal hex digit.
URL encoded
%E2%86%A6
1
On screen
1
Code points
1
JS .length
3
UTF-8 bytes
A visible symbol can contain several code points. JS .length counts UTF-16 units; UTF-8 counts bytes.