Mind you, a lot of bus companies that became part of NBC had three-digit area based numbers for many years previously. This applied to both BET and Tilling group companies - Midland Red was so large they had both three digit area based numbers, and routes with prefix letters, otherwise they'd have run out of numbers altogether. In many cases there was a back-office reason for developing a company wide number system. As long as income and costs were allocated to routes entirely manually it didn't matter if numbers were duplicated as long as they weren't duplicated within a single depot's operations. As comptometers and then early computers came into use with centralised data inputting, there had to be a simple way of avoiding mixing up the data. This became even more important as smaller NBC subsidiaries were taken under the wing of larger ones with shared administration. Now there was always a way round this. Although Ribble had an area numbering scheme they only used the numbers up to the end of the 6nn series. There were also routes with letter prefixes, both town services and schools/works services, as well as the extensive express network. There was a conversion chart for letter prefixed routes and internally these used the 7nn, 8nn and 9nn series, without affecting what was displayed on the bus, in timetable, or on crew duties. In effect this is what still happens nationwide today except that the conversion to distinguish duplicated route numbers is built into the data processing system, and the computer systems don't object to alpha numeric data. Either the ticket machine does the conversion, or it happens when the data is downloaded at the depot into the central systems, usually by adding a depot code field alongside the route number.