28 square miles, but had a population of about 475,000. The 1868 arrangement required a customs cordon all around the city. The customs line was drawn according as the locality facilitated customs control. Although Holstein had been annexed by Prussia in 1866, Altona in Holstein remained outside the Customs Union frontier because it flowed completely with the Hamburg urban area. Already in 1853 Altona was excised from the Danish customs area in Holstein and treated as part of Hamburg for the purposes of raising duty. However, the few and low tariffs of Hanover, Oldenburg, Denmark, and Mecklenburg had caused no difficulties. In 1867 Altona remained with Hamburg for Customs purposes, but now the customs border was that around a protective tariff regime. In 1853, between Ottensen and Altona, a customs border was established, which ran along the later Haubachstrasse This continued under the 1867 arrangement, so Ottensen was in the economic territory of the Customs Union, but not Altona and Hamburg. As a result, Hamburg enterprises who wished to sell into the Customs Union set-up sites in Ottensen, which then developed into a major industrial location. Within 60 years, the population grew sixteen-fold, from 2,411 in 1840, to 37,738 in 1900. Enterprises needing to achieve low prices for the world market, and also servicing the not insignificant demand of the inhabitants of the Hamburg-Altona district (578,000 inhabitants in 1885), would site themselves in the free area. Branches of the same company were often sited both within and without the customs border. The Hamburg-Holstein border cut through houses, roads and gardens and so customs control would have been extremely difficult. This was the reason for outlying areas of Hamburg being incorporated within the customs frontier. Also outside the Customs Union was most of Wandsbek, a salient of Holstein into the Hamburg urban area. End of Hamburg’s own currency Until 1873 Hamburg had its own currency, the Hamburg Mark Courant, divided into 16 schillings, itself divided into 12 pfennig (pence). On 15 February 1873, accounts were converted to the new single currency of the new empire, at a fixed exchange rate of 1.2 imperial Marks to the Hamburg Mark. After a transition period of almost 2 years, the old coins were withdrawn and the new currency became the sole means of payment on 1 January 1875. The senate had delayed the introduction of the new currency after the founding of the empire