History of Derbyshire
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Derbyshire was traditionally divided into six hundreds, namely Appletree, High Peak, Morleyston and Litchurch, Repton and Gresley, Scarsdale, Wirksworth. These were based on the seven earlier wapentakes recorded in the Domesday Book, with the merging of Repton and Gresley wapentakes.
Derbyshire came into existence at the same time as Warwickshire, Leicestershire and Staffordshire (see the histories of each for details) as an administrative division of the Kingdom of Mercia. The actual date for this is unclear but would be in the mid to late 10th Century. Evidence of this division can be seen from the map layout around the village of No Man's Heath where the four counties still effectively meet.
Derbyshire had a detached part in north-western Leicestershire, surrounding Measham and Donisthorpe. This escaped regularisation in 1844, and was incorporated into Leicestershire in 1888 when the county councils were set up. The thin strip of Leicestershire between the exclave and Derbyshire, containing Overseal and Netherseal, is now considered part of Derbyshire.