function openFrame(mainL, rechtsOben, rechtsMitte, rechtsUnten)
{
  if (rechtsOben != '')
    window.frames['links_Oben'].window.location = rechtsOben; 
    
  if (rechtsMitte != '')
    window.frames['links_Mitte'].window.location = rechtsMitte; 
    
  if (rechtsUnten != '')
    window.frames['links_Unten'].window.location = rechtsUnten; 
  
  if (mainL != '')
    window.frames['Main'].window.location = mainL;
    
  return false;
}