of population and "relieve" major industrial cities and other sensitive localities of "hiding kulaks and dangerous political elements" and those "not engaged in labor of social usefulness". The "passportization" process developed gradually involving factories, large, medium, and small cities, settlements, and rural areas, and finally became universal by the mid-1970s. Internal passports were used in the Soviet Union for identification of persons for various purposes. In particular, passports were used to control and monitor the place of residence by means of the propiska, a regulation designed to control the population's internal movement by binding a person to his or her permanent place of residence. For example, a valid propiska was necessary to receive higher education or medical treatment, although these services were not limited to the location registered. Besides marriage to a resident of another area, university education was the most popular way of circumventing one's propiska and residing elsewhere. Also, since only a minority of dwellings were privately owned, having a propiska at a certain address meant that one had the right to live there. All residents were required by law to record their address in the document and to report any relevant changes to a local office of the Ministry of Internal Affairs. For example, citizens needed to submit photographs of themselves for their passport, taken when they were issued the document at age 16, and again at ages 25 and 45. Formally, passports were not necessary for traveling per se in late Soviet Union. Bus, train, and air tickets were sold without names, and identification documents were not necessary for boarding buses and trains (and only became necessary to board a plane in the mid-1970s) except when traveling to/from border-adjacent areas and controlled cities. Nevertheless, passports were necessary for temporary propiska in a number of situations such as checking in a hotel or renting a private dwelling (no marks were placed in the document). Moreover, in the late 1980s and early 1990s, Soviet internal passports, accompanied with a special leaflet, were valid for traveling to most Comecon countries and Yugoslavia as a member of a touristic group. The leaflet functioned as an equivalent of exit visa stamped in international passports; destination countries did not require entry visas at that time. The Russian Federation In 1992, passports, or other photo identification documents, became necessary to board a train. Train tickets started to bear passenger names, allegedly as an