Duke Ernest the Iron and his descendants unilaterally assumed the title "archduke". That title was only officially recognized in 1453 by Emperor Frederick III, himself a Habsburg. Frederick himself used just "Duke of Austria", never "Archduke", until his death in 1493. The title was first granted to Frederick's younger brother, Albert VI of Austria (died 1463), who used it at least from 1458. In 1477, Frederick granted the title "archduke" to his first cousin Sigismund of Austria, ruler of Further Austria. Frederick's son and heir, the future Emperor Maximilian I, apparently only started to use the title after the death of his wife Mary of Burgundy in 1482, as "Archduke" never appears in documents issued jointly by Maximilian and Mary as rulers in the Low Countries (where Maximilian is still titled "Duke of Austria"). The title appears first in documents issued under the joint rule of Maximilian and Philip (his under-age son) in the Low Countries.