A support fixed to hold a thing steady, and a mark that binds a group together. Both hold what would otherwise spread.
Old French brace/brasse (the two arms, an armful), from Latin bracchia, plural of bracchium (arm); a brace was first the clasp of two arms before it named any support or paired mark holding something from both sides.
A support fixed to hold something steady, and a mark that binds a group into one — both hold together what would otherwise come apart or spread.