친구 (chin'gu) usually implies an equality in the relationship. Due to the strict hierarchy of contemporary Korean society, 친구 (chin'gu) tends to be used mainly for people of equal age, i.e. people who were born in the same year; or equal hierarchical status, e.g. same grade in a school. If a tenth-grader is friends with an eleventh-grader, the former will usually refer to the latter as 친한선배(chinhan seonbae, “upperclassmen [I'm] friendly with”) rather than as 친구 (chin'gu).
Before the division of Korea in 1945 and the installment of a Communist government in North Korea, the common word for "friend" was 동무(dongmu). 친구 (chin'gu) had formal overtones and was not generally used by children. However, the North Korean government used 동무(dongmu) as its translation for "comrade," which made the word ideologically unwelcome in South Korea following the Korean War. 동무(dongmu) was thus replaced by 친구 (chin'gu) in South Korea.