What ∬ means
It totals values across a two-dimensional region.
In ∬ f(x,y) dA, f gives the value at each point and dA a tiny area; with f=1, the result is the region’s area.
It extends ordinary integration: surface height across the region gives volume, a volume integral. An iterated integral performs both integrations in sequence, one variable at a time; changing order can matter. In the eighteenth century, mathematician Leonhard Euler used it for volumes and surface areas.
What it means when someone sends it to you
2 readings∬ 1 dA
Find the region’s area.
∬ ρ(x,y) dA
Total density across an area, such as mass.
Don't confuse it with
4Questions 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 222C, 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 “double integral”.
Linux
Ctrl+Shift+UType 222c, then Enter or Space.
HTML
∬CSS
\222CMicrosoft Word
In equations, type \iint or \doubleint.
Linux
Press Ctrl+Shift+U, enter 222C, then Enter.
macOS
Enable Unicode Hex Input; hold Option and type 222C.
Codes and encoding1 code points
HTML decimal
∬
HTML hex
∬
CSS content
content: "\222C";
Each escape is closed by the next backslash; a trailing space is needed before a literal hex digit.
URL encoded
%E2%88%AC
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.