Used to add a comment to the right of and encompassing one or more lines, or to indicate that items to the left are subdivisions of the item on the right. Compare {.
A ⎫
B ⎬ these are a few letters of the alphabet
C ⎭
Joseph Emerson Worcester, A Dictionary of the English Language, volume 1:
† AT'TRY, ⎫ a. [A.S. atter, poison.] Poison-
† AT'TER-LY, ⎭ ous; virulent. Chaucer.
Used to indicate that two or three lines of a poem form a doublet or triplet.
Thomas F. Adams, Typographia; Or, The Printer's Instructor: A Brief Sketch: Braces stand before, and keep together, such articles as are of the same import, and are sub-divisions of the preceding articles. They sometimes stand after, and keep together, such articles as make above one line, and have [...] posts after them, which are justified to answer to the middle of the brace. The bracing side of a brace is always turned to that part of an article which makes the most lines.