An item with internal machine threads designed to fit the external machine threads of a tube fitting. It has flat faces, one of which has a counterbored recess to accommodate packing, packing or sealing rin (s), gasket(s), and the like. The item, when installed, provides a method of locking and sealing the connection. See also locknut, pipe; nut (1), tube coupling and nut (1) , plain, hexagon.