Upper-left or lower-right curly bracket section

A math font uses it to build a tall curly brace.

In the two-piece form, ⎰⎱ makes a left brace and ⎱⎰ a right one. Math typesetting tools call this section lmoustache; a three-row brace uses a separate set of upper, middle, and lower pieces.

How early does a curly brace appear in print?

A documented printed example dates to 1503.

In Gregor Reisch's book Margarita Philosophica, a curly brace links the branches of a stemma, a diagram of relationships. Printers later assembled braces from short straight rules and rounded corners, fitting them to the right-angle grid of movable type.

Should I type it directly in math text?

Usually not; it is a layout piece, not standard brace notation.

Unicode treats this family of brace pieces as compatibility characters: they can appear in data streams made by display or print drivers, but should not be stored as mathematical text. In a LaTeX document using unicode-math, \lmoustache is the command for this character. For a brace that grows around an expression, use your math typesetter's scalable-brace command.