This study describes and analyses the history of money of account systems as an evolution of an economic institution. These systems were used during nine centuries in Frisia, a historic country along the North Sea coast. Because Frisia was situated along this coast near the mouths of major rivers, and because its population originally lived on heightened mounds amidst the coastal waters, early in the Middle Ages the nation was already experienced with shipping and long-distance trade, and thus accustomed to using money. Even more interesting with respect to the history of money is the lack of a central government to execute a monetary policy. It is for this reason that from the beginning, the evolution of the system of money in Frisia occurred organically, from the bottom up. Without feudal lords, mintage was market-controlled by private enterprise. Using tools of economic analysis, and in particular institutional economics, Henstra brings some order in the confusing medieval currency. One of the instruments used to reconstruct the institutional development is what is known as the wergeld-hypothesis - implying constancy in the silver equivalence of the normal wergeld over centuries - which is supported by about 80 examples. This unorthodox approach also renders new insight into sources including the Lex Frisionum (c. 790), the Mid-frisian Synodal Statutes (10th century), the XVII all-Frisian Statutes (11th century), the Fivelingo Wergeld tradition (13th century), the 1323 Treaty of the Upstallisbam, and several others.