The origins of military obligation in England pre-date the establishment of the English state in the 10th century, and can be traced to the 'common burdens' of the Anglo-Saxon period, among which was service in the fyrd, or army. There is evidence that such an obligation existed in the Kingdom of Kent by the end of the 7th century, Mercia in the 8th century and Wessex in the 9th century, and the Burghal Hidage of 911–919 indicates that over 27,000 men could have been raised in the defence of 30 West Saxon boroughs. In the late 10th century, areas began to be divided into 'hundreds' as units for the fyrd. The obligation to serve was placed on landholders, and the Domesday Book indicates that individuals were expected to serve for approximately 60 days.