North Korea has long allowed an unusual degree of private trade in foreign currency for a communist bloc country, and a new study has shone a light on just how common it is.

South Korea’s central bank recently published a survey of North Korean refugees that deals with the payment systems in the North last year and found that nearly one in three defectors who remitted money to family in the DPRK sent dollars or yuan. Those sending cash to Pyongyang preferred dollars, while border city destinations were more likely to receive yuan.