The SO area originally had both single and double-digit postcode districts. From the inception of postcodes in Southampton, "large users" within the SO1 and SO2 postcode areas (which broadly covered the area of the city) were allocated SO9 postcodes. In the early 1990s, a shortage of available postcodes occurred due to increased demand. On 1 December 1993 Royal Mail took the step of dividing each of the existing single-digit districts, the numbers of the new subdivisions containing two digits rather than one. For example, the SO1 and SO2 districts were divided into SO14-19. At the same time, ordinary "geographical" postcodes replaced the SO9 codes. Whilst this created thousands more possible postcode combinations, it resulted in the unusual situation where the centre of the postcode area, Southampton, lacks a district numbered 1. The area is one of only a few in the country where this is the case.
The regional processing centre in Southampton handles incoming and outgoing mail from across the entire PO, SO and SP areas, as well as some outgoing mail from Dorset imported to make best use of Southampton's higher processing capacity.
Coverage
The approximate coverage of the postcode districts:
