This paper is concerned with Esperanto, the only planned language system that has managed the successful transition from the status of a mere project to a full-fledged language. This is partly due to linguostructural properties, but above all to extralinguistic factors. Esperanto has found a sufficiently diverse and productive speech community which guarantees the constant and sustained dissemination of the language. This paper describes Esperanto as a planned language of the autonomous a posteriori subgroup. Its linguistic norm, which is documented in the Fundamento de Esperanto (1905), has developed steadily and become stable. It is supervised by the Akademio de Esperanto. Self-regulation takes place in a field of tension between diversifying forces (e.g. different linguistic and cultural influences because of the speakers' native backgrounds) and unifying forces (e.g. application during international meetings, literature, and radio programs). Its productive and flexible word formation system and syntax make Esperanto a means of communication with high expressive quality and stylistic variation. The main reason for the ease of Esperanto communication, however, is its democratic character: the lack of a native speaker whose competence decides on the language standard facilitates symmetrical communication.