Historically, telephones were typically owned by the telephone company, and were usually permanently wired to the telephone line. For some applications it was necessary or convenient to provide portable telephone sets that could be moved to a different location within the customer's premises. For this purpose telephone companies developed jacks and plugs in various designs with various numbers of contacts. Before c. 1930, concentric connectors with three contacts were usually sufficient, but the upgrade of telephone sets to anti-sidetone circuitry in the 1930s required at least four conductors between a desk set and the subscriber set that contained the telephone hybrid and a ringer. For this purpose, Bell System engineers developed a cube-shaped four-prong plug (type no. 283) with uneven prong spacings to avoid improper insertion into the jack.