What ∷ means
A:B∷C:D reads “A is to B as C is to D.” The relationship between A and B matches the one between C and D. The ratio sign ∶ separates the terms in each pair; ∷ joins the pairs.
The notation developed over time. Oughtred's proportion notation appears in work composed about 1628; Vincent Wing used A:B∷C:D in 1651. A Unicode proposal attributes the compact form ∷ to Leibniz in 1696. Unicode material also distinguishes ∷ from punctuation formed by four dots in a square.
Don't confuse it with
1Questions 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 2237, 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 “proportion”.
Linux
Ctrl+Shift+UType 2237, then Enter or Space.
HTML
∷CSS
\2237Codes and encoding1 code points
HTML decimal
∷
HTML hex
∷
CSS content
content: "\2237";
Each escape is closed by the next backslash; a trailing space is needed before a literal hex digit.
URL encoded
%E2%88%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.