At the top of each card, there is a stamped prisoner number which gave the number card file its name. Each concentration camp prisoner was assigned a number, known as their prisoner number, when they arrived at a camp. In the concentration camps, these numbers were more important than the prisoners’ names. During roll call, for example, prisoners had to report with their number instead of their given name. Sometimes the same number was assigned multiple times in the camps – after large numbers of prisoners had been released or transferred, for example, or when prisoners died. Furthermore, prisoners almost always received a new number when they were transferred to a different camp. An early overview of the prisoner numbers from 14 concentration camps can be found here.

The numbers on these cards from Buchenwald were stamped with the same stamp that was used to apply the numbers to the strips of fabric that prisoners had to sew onto their clothing.