South Jutland — a fief of the DANISH crown, not of the Empire, erected as a duchy for Abel, younger son of Valdemar II. From 1460 its dukes were the same men as the counts (later dukes) of Holstein, an 'up ewig ungedeelt' union that welded a Danish fief to a German one and produced the nineteenth century's most notorious diplomatic tangle.
Ceded by Denmark to Austria and Prussia at the Treaty of Vienna, 30 October 1864; annexed by Prussia in 1866 and legally merged into the Province of Schleswig-Holstein. North Schleswig voted to rejoin Denmark in the 1920 plebiscite.
The standard English-language monograph on the duchies' constitutional status and the 1863-64 crisis (Harvard University Press).
Explains why Holstein was in the Empire and Schleswig was not — the distinction this record encodes.