Editing Postcodes in the United Kingdom (section)

=== Postcode districts and numbering ===
Each postcode area contains a number of post towns and postcode districts.  All districts are defined by either one or two digits (AA9 or AA99); in London (only), some districts are additionally defined by one digit and one letter (A9A or AA9A).

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 (central Birmingham) – but there a limited number of postcode areas that start 0 or 10, e.g. SL0 and AB10.  Croydon uniquely has no CR1 despite having CR0 and CR2-CR10 (an unintended outcome from the initial pilot of three letter postcode areas, where "CRO" for Croydon was transferred to the new format and became CR0, rather than CR1).  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 a different pattern -

* '''geographical direction''' (e.g. the Outer Hebrides postcode area HS, where the districts are numbered from north to south)
* '''alphabetical order''' of district sub-offices, which is used only in London postcode areas (e.g. E2 is Bethnal Green, E3 is Bow, E4 is Chingford etc., which results in intuitive anomalies such as SE1 and SE2 being far from neighbours, with SE1 being large part of Central London south of the Thames (Waterloo and the borough of Southwark) and SE2 covering Abbey Wood at the far eastern end of the Elizabeth Line.
* '''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)
* '''earlier districts''' where preceding postal districts have been adopted or translated into the current system (such as Glasgow's "compass points") - see Earlier Postal Districts.

Numbering of postcode districts is normally consecutive, starting from "aa1" (e.g. Halifax which has seven districts numbered consecutively HX1 to HX7), but this is not universal.  Non-consecutive numbering can arise when geographic realities are faced and consecutive numbering would be potentially misleading or unhelpful, or when new districts are created or added/removed from the postcode area.

* '''geographic reality''': particularly in larger postcode areas, the scale of the districts prevents consecutive numbering e.g. NE postcode area's NE50-PH60 are not allocated due to the substantial distance between NE49 (Haltwhistle) and the next district to be numbered (Morpeth, which was numbered NE61 rather than NE50), recognising the geographic separation of districts in the west (up to NE49) from those to the north (NE61 upwards).
* '''new postcode districts:''' new postcode districts are occasionally created, usually due to increased demand for addresses following housing/business development and exhaustion of available postcodes for the existing district. There appear to be no clear rules for numbering new districts.  If no sequential numbers are available, then new non-consecutive numbers are allocated.  For example, in the [[CV postcode area]], the CV47 district was formed in 1999 from parts of the CV23 and CV33 districts, where the highest numbered district then allocated was CV37.
* '''transfer of postcode districts''':  postcodes districts can be transferred for operational reasons e.g. PH49 and PH50 postcode districts which resulted from the transfer and recoding of PA39 and PA40 districts (PH39 and PH40 were already allocated although PH45-PH48 were not allocated).  This also occurred with the creation of the HS postcode area, the only new geographic postcode area created since 1973, from the previous PA80 to PA87 districts.

London uniquely uses letters for subdivisions of some of its postcode districts (E1, N1, W1, WC1, WC2, EC1-EC4, NW1, SE1, SW1) with letters used substantively in three areas: W (W1), WC (WC1 and WC2) and EC (EC1-EC4), these being the most central areas with the densest concentration of addresses. These are effectively postcode districts in their own right, and could have been given double digit numbers (e.g. for WC in the range WC10-WC19 and WC20-WC29).  The districts are geographically extremely small.  Using double digit numbering may have encountered resistance due to their "micro-size" and also due to the long-standing use of "WC1" etc. which signified in some cases a cultural or wealth status which may have been diluted or lost if re-coded from WC1 to, say, WC15.  Even if London had dropped the initial compass points systems to create a single post code area, the challenge of the large number of districts (169) would require a bespoke numbering system:
{| class="wikitable"
|+
!Area
!Geo. Districts (total N/NN/aN)
!(aN combination)
!Non-geo. Districts (total) 
!(aN combination)
|-
|E
|20
|1
|2
|0
|-
|EC
|23
|23
|4
|3
|-
|N
|23
|1
|2
|1
|-
|NW
|11
|0
|2
|1
|-
|SE
|28
|0
|1
|1
|-
|SW
|27
|8
|0
|0
|-
|W
|25
|12
|1
|1
|-
|TOTAL
|157
|45
|12
|7
|}
Accordingly, many postcode districts are not physically contiguous, despite the inference from their numbering.  Likewise, the centrality of a postcode district within a postcode area cannot be reliably inferred from the postcode alone. See [[List of postcode areas in the United Kingdom|postcode area]].