Ravensbrück concentration camp
Prisoners had to sew their serial numbers onto their clothes.  They also had to wear colored triangles which showed why they were sent to Ravensbrück.  In the middle of the triangle, they had to sew a letter saying what country they were from.  For example, Polish prisoners had to sew a red triangle with the letter "P" (for Poland) onto their clothes.  (A red triangle showed that a person was a political enemy of Nazi Germany.)