Daily (also at weekends and on public holidays) from start to end of service travel is free of charge on the tram lines and the replacement bus services of the lines 1, 3, 4, 5, 6, 7 and 23. Free travel is available between the following stations:
Lines 1 + 7: between Südtiroler Platz + Kaiser-Josef-Platz
Lines 3 + 5: between Schlossbergplatz + Finanzamt or Dietrichsteinplatz
Lines 4+ 6: between Südtiroler Platz + Finanzamt or Dietrichsteinplatz
Lines 23: between Jakominiplatz + Dietrichsteinplatz
You can find more information about the Altstadtbim here: Altstadtbim I The Graz old town tram