A device for supporting a patient's arm includes a rail clamp configured to be secured to a rail or other structure on a table such as an operating table. The rail clamp includes a clamp body and a socket configured to receive a base on an attachment such as a lateral brace. The base may be inserted longitudinally in the longitudinal socket on the rail clamp. The base may comprise an arm brace. The base is moveable in the longitudinal socket such that the arm brace may be raised or lowered relative to the rail clamp. The arm brace may be used when a patient is lying prone or laterally on a table.