Curly Bracket Extension

It extends a curly brace across multiple rows. A typesetter places it between the brace’s upper, middle, and lower pieces, repeating the extension when a formula needs more height.

The neighboring pieces supply the curves: ⎧, ⎨, and ⎩ form a tall left curly brace; ⎫, ⎬, and ⎭ form its right-hand counterpart. Unicode specifies ⎪ in five-row versions of both constructions.

In 1998, a Unicode proposal traced these brace pieces to characters already used in TeX, PostScript, HP Math8, Microsoft Symbol, and DEC VT—typesetting, printer, or character-encoding environments that predated Unicode. They let mathematical software assemble tall braces from pieces instead of stretching an ordinary brace.

How do I type ⎪ in math markup?

Use your typesetting system’s brace-extender command. In unicode-math, the command is \vbraceextender; groff uses the glyph name \[braceex].

Unicode 17.0.0 treats these pieces as compatibility characters and says not to use them in stored mathematical text. Put a scalable-brace command in the math source instead of saving a stack of ⎪ characters.