 In Friedrichshafen on 26 August 1873 Karl August married Princess Pauline of Saxe-Weimar-Eisenach.They were second cousins, as she was the paternal granddaughter of Prince Bernhard, who had been in the service of the king of the Netherlands and was a younger brother of the Grand Duke Karl Frederick of Saxe-Weimar-Eisenach, the grandfather of Karl August.Her maternal grandparents were William I of Württemberg and his third wife Pauline Therese.