text: The kingdom included the Middle Saxon Province, which included the area of the later county of Middlesex, and most if not all of Hertfordshire Although the province is only ever recorded as a part of the East Saxon kingdom, charter evidence shows that it was not part of their core territory. In the core area they granted charters freely, but further west they did so while also making reference to their Mercian overlords. At times, Essex was ruled jointly by co-Kings, and it thought that the Middle Saxon Province is likely to have been the domain of one of these co-kings. The links to Essex between Middlesex and parts of Hertfordshire were long reflected in the Diocese of London, re-established in 604 as the East Saxon see, and its boundaries continued to be based on the Kingdom of Essex until the nineteenth century.
  The East Saxons also had intermittent control of Surrey. For a brief period in the 8th century, the Kingdom of Essex controlled west Kent.
  The modern English county of Essex maintains the historic northern and the southern borders, but only covers the territory east of the River Lea, the other parts being lost to neighbouring Mercia during the 8th century.
  In the Tribal Hidage it is listed as containing 7,000 hides.