A guide for students at the University of Trier gives the correct written forms: Sehr geehrter Herr Professor Knodel and Sehr geehrte Frau Dr. Freudenreich. It adds that many a chief physician would not let even a patient call him merely Herr Doktor instead of Herr Professor. A guide on e-mail etiquette used at Goethe University Frankfurt tells students: Wenn Ihre Lehrenden akademische Titel führen, sollten Sie sie verwenden. If your teachers hold academic titles, you should use them. A first name is appropriate only once the addressee has explicitly offered the du. Erst dann ist die informelle Variante Liebe/r <Vorname> angemessen. Only then is the informal form Dear followed by the first name appropriate. The doctorate even appears on the German identity card. The identity card law lists the Doktorgrad, the doctoral degree, among the few details printed on the card.