Above all it evolved the Customs-Union (_Zollverein_), which gradually attached the smaller states, by material interests if not by sympathy, to the Prussian system. A reform of the tariff conditions in the new Prussian monarchy had been from the first a matter of urgent necessity, and this was undertaken under the auspices of Baron Heinrich von Bülow (1792-1846), minister in the foreign department for commerce and shipping, and Karl Georg Maassen (1769-1834), the minister of finance. When they took office there were in Prussia sixty different tariffs, with a total of nearly 2800 classes of taxable goods: in some parts importation was free, or all but free; in others there was absolute prohibition, or duties so heavy as to amount to practical prohibition. Moreover, the long and broken line of the Prussian frontier, together with the numerous enclaves, made the effective enforcement of a high tariff impossible. In these circumstances it was decided to introduce a system of comparative free trade; raw materials were admitted free; a uniform import of 10% was levied on manufactured goods, and 20% on "colonial wares," the tax being determined not by the estimated value, but by the weight of the articles. It was soon realized, however, that to make this system complete the neighbouring states must be drawn into it; and a beginning was made with those which were enclaves in Prussian territory, of which there were no less than thirteen. Under the new tariff laws light transit dues were imposed on goods passing through Prussia; and it was easy to bring pressure to bear on states completely surrounded by Prussian territory by increasing these dues or, if need were, by forbidding the transit altogether. The small states, though jealous of their sovereign independence, found it impossible to hold out. Schwarzburg-Sondershausen was the first to succumb (1819); Schwarzburg-Rudolstadt (1822), Saxe-Weimar and Anhalt-Bernburg (1823), Lippe-Detmold and Mecklenburg-Schwerin (1826) followed suit so far as their "enclaved" territories were concerned; and in 1826 Anhalt-Dessau and Anhalt-Cöthen, after several years' resistance, joined the Prussian Customs-Union. In 1828 Hesse-Cassel entered into a commercial treaty with Prussia. Meanwhile, alarmed at this tendency, and hopeless of obtaining any general system from the federal diet, the "middle" states had drawn together; by a treaty signed on the 18th of January 1828 Württemberg and Bavaria formed a tariff union, which was joined in the following year by the Hohenzollern principalities; and on the 24th of September 1828 was formed the so-called "Middle German Commercial Union" (_Handelsverein_) between Hanover, Hesse-Cassel, the Saxon duchies, Brunswick, Nassau, the principalities of Reuss and Schwarzburg, and the free cities of Frankfort and Bremen, the object of which was to prevent the extension of the Prussian system and, above all, any union of the northern Zollverein with that of Bavaria and Württemberg.