These letters have been used for multiple purposes. Originally, they referred to the leading letters of telephone exchange names. In the mid-20th century United States, before the switch to All-Number Calling, telephone numbers had seven digits including a two-digit prefix which was expressed in letters rather than digits, e.g.; KL5-5445. The UK telephone numbering system used a similar two-letter code after the initial zero to form the first part of the subscriber trunk dialling code for a region. For example, Aylesbury was assigned 0AY6, which translated into 0296.