Original: The Duchy of Saxe-Weimar-Eisenach was created in 1809 by the merger of the Ernestine duchies of Saxe-Weimar and Saxe-Eisenach, which had been in personal union since 1741, when the Saxe-Eisenach line had died out.
Simplified: The two duchies had shared the same ruler since 1741, which is when the Saxe-Eisenach line had died out.