The cracks in the belly of the instrument tend to run parallel to the annual rings of the wood. There can be several reasons for the cracks. The dry air resulting from indoor heating in the winter can cause such tension in the instrument that cracks result. Undue force in handling the instrument can also cause damage. In rare cases a cross fracture can result. On the belly, those cracks that are above the bass bar as well as bass bar cracks along the bar, known as sound post cracks, are particularly dangerous. The repair of such cracks requires great skill and experience. Occasionally it is even necessary to open the instrument in order to glue and secure such cracks with precisely fitted little pine blocks that are placed to bear the forces from the opposite directions to the belly. Sometimes specially prepared strips of vellum are used. The repair of a bass bar requires a special method whereby a lining is worked into the bar from within. It is extremely important that the cracks are properly glued from within and from the outside and that the repair is not later visible. Only soluble glue should be used so that further repairs are possible.