Scheidt & Bachmann Ticket XPress tickets conform with many conventions established at the start of the computerised ticketing era. Each machine is assigned a unique four-digit identity number (the "machine number") which is printed on the ticket. Ticket XPress machines started at number 2000 (at Twickenham) and were subsequently numbered in the 2000- and 3000-series. By November 2011, the highest numbered machine was 3632 at Stratford; a year later, when 1,594 machines were in use on the British railway network, number 3717 was the highest. The software controls the machine number; it is not hard-coded. Every ticket issuing location on the railway network is allocated a National Location Code (NLC)—a four-digit code used for accounting and other purposes. Each Ticket XPress machine is then assigned to the NLC of its base location, and this is also printed on tickets. Also, a "window number" is allocated to each machine. This sequential number distinguishes it from other machines allocated to the same NLC. Window numbering starts at a different point on each ticket issuing system; on the Scheidt & Bachmann Ticket XPress, the first machine at each location is numbered 21, the second is 22 and so on. Machines at several locations have shown incorrect NLCs or window numbers at various times; for example, upon installation London Waterloo's machines showed Basingstoke's NLC, and at Hampton Wick the Ticket XPress bore the code for Strawberry Hill. Most errors have been corrected quickly and may have been caused by errors in programming during installation.