What ⇹ means
It marks a partial relation.
In Z notation, ⇹ is the partial relation symbol. A relation pairs elements from two sets; “partial” means some possible pairs may be absent. The formula identifies those sets.
Formal Z notation; its reading depends on the surrounding expression.
The opposing arrowheads alone do not mean “back and forth.”
Don't confuse it with
4⇷⇸⇺⇻
Leftwards Arrow With Vertical Stroke
shares “arrow”, “stroke”, “vertical”
Rightwards Arrow With Vertical Stroke
shares “arrow”, “stroke”, “vertical”
Leftwards Arrow With Double Vertical Stroke
shares “arrow”, “stroke”, “vertical”
Rightwards Arrow With Double Vertical Stroke
shares “arrow”, “stroke”, “vertical”
Worth knowing
- STIX, a scientific and technical publishing project, submitted a proposal on 2000-01-09 requesting mathematical and technical symbols and incorporating the ISO Standard Z character set.
Questions people ask
1Why does ⇹ show as a box on some devices?
Because that device's fonts have no glyph for it. The character is still correct and still copies correctly — the box is a missing glyph, not a broken character.
How to type ⇹
Windows (Word)
Alt+XType 21F9, 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 “left right arrow with vertical stroke”.
Linux
Ctrl+Shift+UType 21f9, then Enter or Space.
HTML
⇹CSS
\21F9Codes and encoding1 code points
HTML decimal
⇹
HTML hex
⇹
CSS content
content: "\21F9";
Each escape is closed by the next backslash; a trailing space is needed before a literal hex digit.
URL encoded
%E2%87%B9
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.