What ⇖ means
It points northwest; the surrounding notation sets its exact role. As a math arrow, its doubled shaft does not define one universal operation. A diagram may use it as a direction mark.
In calculus, the mathematics of change, arrows for limits—the values functions approach—are documented by 1905 and spread in influential books in 1908. The mathematician Hilbert used an arrow for “if this, then that” in 1922; a double-headed arrow for the two-way version appears about 1932. These are broader arrow milestones, not ⇖’s origin.
Technical in mathematical notation; a diagram may use it as a direction mark.
Do not assume its northwest direction means implication or equivalence.
Don't confuse it with
3Questions 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 ⇖
⇖\21D6Codes and encoding1 code points
A visible symbol can contain several code points. JS .length counts UTF-16 units; UTF-8 counts bytes.