resources of the land itself. Over time the hide became the unit on which all public obligation was assessed; as well as food rent, the manning and maintenance of the walls of a burh and the amount of geld payable was based on the hide. Tenants had a threefold obligation related to their landholding; the so-called ‘common burdens' of military service, fortress work, and bridge repair. Later the hide was given a set acreage and in the Domesday book the most common size in use was . However some areas such as Dorset and Wiltshire used units based on to . In wartime, five hides were expected to provide one fully armed soldier in the king's service, and one man from every hide was to provide garrison duty for the burhs and to help in their initial construction and upkeep. The continued maintenance of the burhs, as well as ongoing garrison duty, was also probably supplied by those inhabitants of the new burhs which were planned by the king as new towns. In this way the economic and military functions of the larger burhs were closely interlinked. The hide also served as a unit of fiscal assessment for the collection of a tax, known as Danegeld, for which the original purpose was to raise money to buy off raiding Vikings; however after that threat had retreated it was retained as a permanent land-tax. Origins of the document The document probably dates from after 914 during the reign of Alfred's son, Edward the Elder. This assumes that it was compiled as part of the preparations for Edward the Elder's campaign against the Danes in 917. The list identifies 30 burhs in Wessex, two in Mercia and one in Hwicce. The view that the Burghal Hidage is of early 10th century date is based on the inclusion of Buckingham and Oxford, two settlements that were sited in Mercia not Wessex, and according to the Anglo-Saxon Chronicle, Buckingham was created as a burh by Edward the Elder in 918. The Chronicle also reports that Edward the Elder took possession of London and Oxford in 910; Buckingham being situated between the two would have also been included. It is possible that the Burghal Hideage was created as a blue-print for the way that burhs were connected with hidation, originally worked out in Wessex, and applied to the situation in Mercia at that time. This