The unit described on this page was in use prior to modern methods of precision measurement. It was based on a standard which is no longer available, and which is not capable of being calibrated against modern measurement units. Therefore, the equivalence to modern SI units or to current United States customary units can only be considered approximate.
The Zoll (generally rendered inch in English) was a Prussian unit of length or distance prior to 1872, when the German Empire was formed, and all traditional local systems were abolished in Germany.
Since no actual Prussian standards are definitively known by the present day, the only way of determining the length of any Prussian unit would be to measure something in modern terms whose length was given by the Prussians in their units. And because this procedure does not give us any clue to which unit may have been the base unit and which were subsidiary units, this distinction really does not apply to the Prussian units here given, so all the units really have equal status. However, most references appear to have treated the Fuss (Prussian foot) as the base unit, expressing other units in terms of this unit. The Zoll, then, can be considered as defined as 1/12Fuss (Prussian foot).