The Duden doesn't define correct spelling, it merely documents it. It's decidedly not a prescriptive work. What you're likely thinking of is the German orthogra

The Duden doesn't define correct spelling, it merely documents it. It's decidedly not a prescriptive work. What you're likely thinking of is the German orthography rules defined by the Rat für deutsche Rechtschreibung and in turn strongly influenced by actual practice rather than just dogma -- the most recent revisions are very forgiving with regard to acceptable variations. Linguists on the other hand have given up on prescriptivism decades ago. Scientific linguistics are entirely descriptivist, studying how language is used rather than prescribing how it ought to be used.