Bedford, Hereford, Northampton, Oxford and Stamford. In addition to their defensive uses, these burhs are thought to have been administrative centres, serving as regional markets and indicating a transformation of the Mercian economy away from its origins as a grouping of midland peoples. The burhs are forerunners of the defensive network successfully implemented by Alfred the Great a century later to deal with the Danish invasions.Alan Vince, "Market Centres and Towns in the Mercian Hegemony", in Brown & Farr, Mercia, p. 192. However, Offa did not necessarily understand the economic changes that came with the burhs, so it is not safe to assume he envisioned all their benefits. In 749, Æthelbald of Mercia had issued a charter that freed ecclesiastical lands from all obligations except the requirement to build forts and bridges – obligations which lay upon everyone, as part of the trinoda necessitas.Campbell, The Anglo-Saxons, p. 100. Offa's Kentish charters show him laying these same burdens on the recipients of his grants there, and this may be a sign that the obligations were being spread outside Mercia.Gareth Williams, "Military Institutions", in Brown & Farr, Mercia, p. 297. These burdens were part of Offa's response to the threat of "the pagan seaman". Offa issued laws in his name, but no details of them have survived. They are known only from a mention by Alfred the Great, in the preface to Alfred's own law code. Alfred says that he has included in his code those laws of Offa, Ine of Wessex and Æthelberht of Kent which he found "most just". The laws may have been an independent lawcode, but it is also possible that Alfred is referring to the report of the legatine mission in 786, which issued statutes that the Mercians undertook to obey. Coinage At the start of the 8th century, sceattas were the primary circulating coinage. These were small silver pennies, which often did not bear the name of either the moneyer or the king for whom they were produced. To contemporaries these were probably known as pennies, and are the coins referred to in the laws of Ine of Wessex.A transcription of Ine's laws can be read at This light coinage (in contrast to the heavier coins minted later in Offa's reign) can probably be dated to the late 760s and early 770s. A second, medium-weight coinage can be identified before the early 790s. These new medium-weight