Duden

The Duden (German pronunciation: [ˈduːdən]) is a dictionary of the German language, first published by Konrad Duden in 1880. The Duden is updated regularly with new editions appearing every four or five years. As of August 2013^[update] it is in its 26th edition it is printed as 12 volumes with each volume covering different aspects of the German language such as loanwords, etymology, pronunciation, synonyms, etc.

The first of these volumes, Die deutsche Rechtschreibung (English: German Orthography), has long been the prescriptive source for German language spelling. The Duden has become the predominant language resource of the German language, stating the definitive set of rules regarding grammar, spelling and use of German language.

In 1872, Konrad Duden, headmaster of a Gymnasium (secondary school) in Schleiz, now in Thuringia, published a German dictionary called the Schleizer Duden, the first Duden. In 1880 he published the Vollständiges Orthographisches Wörterbuch der deutschen Sprache (Complete Orthographical Dictionary of the German Language); this seminal treatise was declared the official source for correct spelling in the administration of Prussia the same year. The first edition of the Duden contained 28,000 entries.

...
Wikipedia