In order to join two tubes when the tubes are not coaxial it is necessary to either use a special coupling or to bend the tubes. Couplings are standard that allow joints to be made at different standardized angles, typically 30.degree., 45.degree., 60.degree., 90.degree., and 180.degree., but when intermediate angles must be made the installer is normally forced to bend the tubes or make up a special coupling.
In German patent document 3,914,420 filed May 1, 1989 by A. W. Sommer the coupling ball is hollow and is formed with an array of holes. Spreadable pins are inserted into the tube ends and then into the appropriate holes and the unused holes are capped. This arrangement provides a wide range of positions, but still does not accommodate intermediate angular settings. In addition the capped holes are obvious and are often considered ugly.
German patent document 4,336,401 describes a system where two part-spherical parts are joined together at a diametral plane and each have a mounting pin that is fitted into a respective tube end. When the two tubes are coaxial the diametral plane forms an acute angle to the axis. Thus when an angle needs to be formed, the two coupling parts can be rotated relative to each other so their pins are not coaxial but instead their axes meet at a center of the sphere formed by the two parts. Such a system does indeed allow many angles to be accommodated, but has the disadvantage that the joint in the coupling ball is visible. Furthermore it is fairly difficult to join the two parts together in a manner where the attachment means is not visible.