concerned with the status of English pilgrims on the continent and with diplomatic gifts, but it reveals much about the relations between the English and the Franks. Charlemagne refers to Offa as his "brother", and mentions trade in black stones, sent from the continent to England, and cloaks (or possibly cloths), traded from England to the Franks. Charlemagne's letter also refers to exiles from England, naming Odberht, who was almost certainly the same person as Eadberht Praen, among them. Egbert of Wessex was another refugee from Offa who took shelter at the Frankish court. It is clear that Charlemagne's policy included support for elements opposed to Offa; in addition to sheltering Egbert and Eadberht he also sent gifts to Æthelred I of Northumbria. Events in southern Britain to 796 have sometimes been portrayed as a struggle between Offa and Charlemagne, but the disparity in their power was enormous. By 796 Charlemagne had become master of an empire which stretched from the Atlantic Ocean to the Great Hungarian Plain, and Offa and then Coenwulf were clearly minor figures by comparison. Government The nature of Mercian kingship is not clear from the limited surviving sources. There are two main theories regarding the ancestry of Mercian kings of this period. One is that descendants of different lines of the royal family competed for the throne. In the mid-7th century, for example, Penda had placed royal kinsmen in control of conquered provinces. Alternatively, it may be that a number of kin-groups with local power-bases may have competed for the succession. The sub-kingdoms of the Hwicce, the Tomsæte and the unidentified Gaini are examples of such power-bases. Marriage alliances could also have played a part. Competing magnates, those called in charters "dux" or "princeps" (that is, leaders), may have brought the kings to power. In this model, the Mercian kings are little more than leading noblemen. Offa seems to have attempted to increase the stability of Mercian kingship, both by the elimination of dynastic rivals to his son Ecgfrith, and the reduction in status of his subject kings, sometimes to the rank of ealdorman. He was ultimately unsuccessful, however; Ecgfrith only survived in power for a few months, and 9th century Mercia continued to draw its kings from multiple dynastic lines. There is evidence that Offa constructed a series of defensive burhs, or fortified towns; the locations are not generally agreed on but may include