What ⟷ means
It asserts a two-way relation. Logic uses it when each statement implies the other; a mapping uses it for a reversible correspondence. In a proof, the step needs justification in reverse too.
Why is it long?
The LaTeX command \longleftrightarrow names the long form used in mapping notation.
MathML, a format for laying out formulas, may stretch its shaft to fit neighboring expressions.
Formal, academic notation.
Both heads claim the reverse direction too.
What it means when someone sends it to you
2 readingsBetween propositions
Each implies the other.
Between mapped objects
The correspondence runs both ways.
Don't confuse it with
2Combination ideas
2P and Q imply each other.
Each step preserves equivalence both ways.
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 27F7, 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 “long left right arrow”.
Linux
Ctrl+Shift+UType 27f7, then Enter or Space.
HTML
⟷CSS
\27F7LaTeX
Use `\longleftrightarrow` in math mode.
HTML
Use `⟷` or `⟷`.
Codes and encoding1 code points
HTML decimal
⟷
HTML hex
⟷
CSS content
content: "\27F7";
Each escape is closed by the next backslash; a trailing space is needed before a literal hex digit.
URL encoded
%E2%9F%B7
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.