text: As a general rule, the central part of the town/city the postcode area is named after will have the number 1 e.g. B1 (Birmingham) – but there a limited number of postcode areas that start 0 or 10, e.g. SL0 and AB10.  Large post towns are generally numbered from the centre outward such that outlying parts have higher numbered districts.  In most post towns, the postcode "aa1 1AA" was allocated to the crown or principal post office.  These are increasingly defunct, as post offices have closed or moved.
  Alternatively, but less commonly, post towns and postcode districts within the area may be numbered according to: geographical direction (e.g. the Outer Hebrides area HS, the districts are numbered from north to south); alphabetical order of district sub-offices (particularly in London (e.g. E2 is Bethnal Green, E3 is Bow, E4 is Chingford etc.); and clusters, particularly if the postcode area encompasses several major towns or cities (e.g. S postcode area with Sheffield S1-S36, Chesterfield S40-S49, Rotherham S60-S65, Barnsley S70-S75 and Worksop S80-S81). Accordingly, the centrality of a postcode district within a postcode area cannot be reliably inferred from the postcode alone (e.g. SE1 covers a large part of Central London south of the Thames whereas SE2 covers Abbey Wood at the far eastern end of the Elizabeth Line). See postcode area.
  Numbering of postcode districts is normally consecutive, but this is not universal. Non-consecutive numbering arises when districts are numbered for geographical and/or clustering reasons (e.g. PH postcode area's highest numbered district is PH50 but, within the range of PH1-PH50, the numbers PH27-PH29 and PH45-PH48 are not allocated, so as to maintain a distinct geographical link for each group of allocated numbers).