A car running on studded tyres must carry a distinguishing sign stuck on at the rear — a white equilateral triangle with a red border and a tyre-stud symbol in the middle. It goes on the back only, not the front.
KET reference: KET appendix 4, 5: a car with studded tyres must have a distinguishing sign stuck on at the rear — a white equilateral triangle with a red border and a tyre-stud symbol in the middle.