Unlike the partly self-governing boroughs that covered urban areas, the counties of medieval England existed primarily as a means of enforcing central government power, enabling monarchs to exercise control over local areas through their chosen representatives – originally Sheriffs and later the Lord Lieutenants – and their subordinate justices of the peace. Counties were used initially for the administration of justice, collection of taxes and organisation of the military, and later for local government and electing parliamentary representation. They continue to form the basis of modern local government in many parts of the country away from the main urban areas, although sometimes with considerably altered boundaries.
The name of a county often gives a clue to how it was formed, either as a division that took its name from a centre of administration, an ancient kingdom, or an area occupied by an ethnic group. The majority of English counties are in the first category, with the name formed by combining the central town with the suffix "-shire", for example Yorkshire. Former kingdoms, which became earldoms in the united England did not feature this formulation; so for Kent, the former kingdom of the Jutes, "Kentshire" was not used. Counties ending in the suffix "-sex" are also in this category and are former Saxon kingdoms. Many of these names are formed from compass directions. The third category includes counties such as Cornwall and Devon where the name corresponds to the tribes who inhabited the area. County Durham is anomalous in terms of naming and origin, not falling into any of the three categories. Instead it was a diocese that was turned into the County Palatine of Durham, ruled by the Bishop of Durham. The expected form would otherwise be "Durhamshire", but it was rarely used.
There are customary abbreviations for many of the counties. In most cases these consist of simple truncation, usually with an "s" at the end signifying "shire", such as "Berks" for Berkshire or "Bucks" for Buckinghamshire. Some abbreviations are not obvious, such as "Salop" for Shropshire, from the Norman-derived word for its county town Shrewsbury; "Oxon" for Oxfordshire, from Latin Oxonium (referring to both the county and the city of Oxford); "Hants" for Hampshire; and "Northants" for Northamptonshire. Counties were often prefixed with "County of" in official contexts, such as "County of Kent". Those counties named after central towns lost the -"shire" suffix, for example Yorkshire would be known as "County of York". This usage was sometimes followed even where there was no town by that name, such as the "County of Berks". The "-shire" suffix was also appended for some counties, such as "Devonshire", "Dorsetshire" and "Somersetshire", despite their origin. There is still a Duke of Devonshire.
Great Britain was first divided into administrative areas by the Romans, most likely following major geographical features such as rivers. Before their arrival there were distinct tribal areas, but they were in a constant state of flux as territory was gained and lost. After the demise of Roman Britain around 410 these first divisions of land were generally abandoned, although traditional divisions taking the form of petty kingdoms such as Powys, Dumnonia and Elmet, remained in those areas which remained British, such as south west England. The areas that would later form the English counties started to take shape soon afterwards, with the Kingdom of Kent founded by settlers around 445. In southern England more widely, shires emerged from earlier sub-kingdoms as part of the administrative structure of Wessex, which then imposed its system of shires, boroughs (or burhs) and ealdormen on Mercia after it came under West Saxon control during the 9th century. Once the Kingdom of England was united as a whole in 927 it became necessary to subdivide it for administrative convenience and to this end, earldoms were created out of the earlier kingdoms. The whole kingdom was divided into shires by the time of the Norman conquest. Robert of Gloucester accounts for thirty-five shires and William of Malmesbury thirty-two, Henry of Huntingdon, thirty-seven. In most cases the counties or shires in medieval times were administered by a sheriff (derived from "shire-reeve") on behalf of the monarch. After the Norman conquest the sheriff was replaced and the shires became counties, or "areas under the control of a count", in the French manner.
Although all of England was divided into shires by the time of the Norman conquest, some counties were formed considerably later, up to the 16th century. Because of their differing origins the counties varied considerably in size. The county boundaries were fairly static between the 16th century Laws in Wales acts and the Local Government Act 1888. Each shire was responsible for gathering taxes for the central government; for local defence; and for justice, through assize courts.