The Agri Decumates or Decumates Agri (Decumatian Fields) were a region of the Roman Empire's provinces of Germania Superior and Raetia; covering the Black Forest, Swabian Jura, and Franconian Jura areas between the Rhine, Main, and Danube rivers; in present southwestern Germany, including present Frankfurt, Stuttgart, Freiburg im Breisgau, and Weißenburg in Bayern. The only ancient reference to the name comes from Tacitus' book Germania (chapter 29). However the later geographer Claudius Ptolemy does mention the desert of the Helvetians in this area.