Mauthausen was one of the first massive concentration camp complexes in Nazi Germany, and the last to be liberated by the Allies. The two largest camps, Mauthausen and Gusen I, were classed as "Grade III" ("Stufe III") concentration camps, which meant that they were intended to be the toughest camps for the "incorrigible political enemies of the Reich". Mauthausen never lost this "Stufe III" classification. In the offices of the Reich Main Security Office ("Reichssicherheitshauptamt"; RSHA) it was referred to by the nickname "Knochenmühle" – the bone-grinder (literally "bone-mill").