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There are many definitions of a "county" with respect Great Britain, below I have compiled a summary of the different counties that exist, both current and historical.
The boundaries of these counties were established over centuries, and influences can be traced back to the Roman occupation. Although not used today, their influence on modern divisions are quite clear.
The registration counties were used for the compilation of census data during this time. Their boundaries were based on Registration Districts, which were in turn based on the locations of workhouses. As such their boundaries did not coincide with those of traditional boundaries.
These county divisions were created for local governance. In some cases, this involved splitting Traditional Counties along the borders that were used by the courts of Quarter Sessions.
In 1974 the Administrative counties as they were then known, were abolished and replaced with the Metropolitan & Non-Metropolitan counties that still stand today (although they have changed considerably during this time). The boundaries of these divisions were based largely on the previous administrative counties, although some smaller counties were merged with larger neighbours.
The boundaries of these counties differ from those above, as they were maintained for the sole purpose of efficiently delivering mail. It would seem reasonable to suppose that at one time they reassembled some other form of county division, but migrated away from them. One of these times was 1974 when Administrative counties were abolished, and replaced with Metropolitan & Non-Metropolitan Counties. Royal Mail adopted some of these changes, but not all of them as it was felt it could give rise to confusion. Postal counties are no longer used by Royal Mail, and are being phased out of their PAF database and are now included in a supplementary "alias" file. This file is to be discontinued in 2013.
These counties have existed for some time in various different forms, and have been redefined several times. In 1888 they were brought into line with the Administrative Counties, in 1974 they were redefined with respect to Metropolitan & Non-Metropolitan Counties. In 1996, further changes to Metropolitan & Non-Metropolitan Counties resulted, once again, of a distinction between local government counties and the ceremonial or geographic counties used for Lieutenancy. Today, ceremonial counties are made up from Metropolitan Counties, Non-Metropolitan Counties, and Unitary Authorities.
These counties were used for land registration.
The counties of Scotland have been evolving since at least 1889
The boundaries of these divisions were generally very different from the counties that they replaced.
These divisions completely replaced the system of Regions and Districts that had been in place since 1975
These counties were broadly based on Pre 1975 Scottish counties. Postal counties are no longer used by Royal Mail, and are being phased out of their PAF database and are now included in a supplementary "alias" file. This file is to be discontinued in 2013.
These boundaries are used for the ceremonial lord-lieutenants.
These counties date back to King Edward the firsts conquest.
These counties/ unitary authorities replaced the previous 8 counties that preceded them.
Postal counties are no longer used by Royal Mail, and are being phased out of their PAF database and are now included in a supplementary "alias" file. This file is to be discontinued in 2013.
The preserved counties of Wales are the current areas used in Wales for the ceremonial purposes of Lieutenancy. These counties replaced the 8 counties that were created in 1974, and were based on their areas.