What ⊺ means
It marks insertion; the author’s notation defines the exact operation.
The English verb intercalate was first recorded in the 1610s for adding a day to a calendar; by 1824 it also meant inserting something between other things. This word history explains the name, not a universal rule for the symbol.
In 1965, Dominique Foata introduced an intercalation product in combinatorics. It extends results about permutations to multisets, collections in which values can repeat.
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 22BA, 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 “intercalate”.
Linux
Ctrl+Shift+UType 22ba, then Enter or Space.
HTML
⊺CSS
\22BALaTeX
Add \usepackage{amssymb} in the document preamble, then write \intercal in math mode.
Codes and encoding1 code points
HTML decimal
⊺
HTML hex
⊺
CSS content
content: "\22BA";
Each escape is closed by the next backslash; a trailing space is needed before a literal hex digit.
URL encoded
%E2%8A%BA
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.