accomplishments." Between 1816 and 1819 he reorganized the commission's collection of antiquities. In 1819 he opened the first Museum of Northern Antiquities, in Copenhagen, in a former monastery, to house the collections. It later became the National Museum. Like the other antiquarians Thomsen undoubtedly knew of the three-age model of prehistory through the works of Lucretius, the Dane Vedel Simonsen, Montfaucon and Mahudel. Sorting the material in the collection chronologically he mapped out which kinds of artefacts co-occurred in deposits and which did not, as this arrangement would allow him to discern any trends that were exclusive to certain periods. In this way he discovered that stone tools did not co-occur with bronze or iron in the earliest deposits while subsequently bronze did not co-occur with iron – so that three periods could be defined by their available materials, stone, bronze and iron. To Thomsen the find circumstances were the key to dating. In 1821 he wrote in a letter to fellow prehistorian Schröder: nothing is more important than to point out that hitherto we have not paid enough attention to what was found together. and in 1822: we still do not know enough about most of the antiquities either; ... only future archaeologists may be able to decide, but they will never be able to do so if they do not observe what things are found together and our collections are not brought to a greater degree of perfection. This analysis emphasizing co-occurrence and systematic attention to archaeological context allowed Thomsen to build a chronological framework of the materials in the collection and to classify new finds in relation to the established chronology, even without much knowledge of their provenience. In this way, Thomsen's system was a true chronological system rather than an evolutionary or technological system. Exactly when his chronology was reasonably well established is not clear, but by 1825 visitors to the museum were being instructed in his methods. In that year also he wrote to J.G.G. Büsching: To put artifacts in their proper context I consider it most important to pay attention to the chronological sequence, and I believe that the old idea of first stone, then copper, and finally iron, appears to be ever more firmly established as far as Scandinavia is concerned. By 1831 Thomsen was so certain of the utility of his methods that he circulated a pamphlet, "Scandinavian Artefacts and Their Preservation,