What ℄ means
It marks the centre line or axis in a technical drawing.
A centre line, also written centerline, is an imaginary reference through the centre of a part or feature. The mark can show symmetry, locate a circle’s centre, align a route, or give dimensions a shared starting line.
A drafting handbook from 1874 describes drawing centre lines first for symmetrical figures, then measuring working dimensions from them. That is the practical role this symbol signals: establish a reference before placing other measurements.
Why does ℄ look like a box?
The font or app may not include a glyph for ℄.
That changes how it is displayed, not which character is present. In a drawing app, a text style that supports the symbol may make it visible.
Technical drafting notation: it marks a reference axis, not decoration.
Outside a technical drawing, its specific meaning may not be clear.
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 ℄
℄\2104Codes and encoding1 code points
A visible symbol can contain several code points. JS .length counts UTF-16 units; UTF-8 counts bytes.