Roman commerce was a major sector of the Roman economy during the later generations of the Republic and throughout most of the imperial period. Fashions and trends in historiography and in popular culture have tended to neglect the economic basis of the empire in favor of the lingua franca of Latin and the exploits of the Roman legions. The language and the legions were supported by trade and were part of its backbone. The Romans were businessmen, and the longevity of their empire was caused by their commercial trade. Whereas in theory members of the Roman Senate and their sons were restricted when engaging in trade, the members of the equestrian order were involved in businesses despite their upper-class values, which laid the emphasis on military pursuits and leisure activities. Plebeians and freedmen held shop or manned stalls at markets, and vast numbers of slaves did most of the hard work. The slaves were themselves also the subject of commercial transactions. Probably because of their high proportion in society compared to that in Classical Greece, the reality of runaways, and the Servile Wars and minor uprisings, they gave a distinct flavor to Roman commerce. The intricate, complex, and extensive accounting of Roman trade was conducted with counting boards and the Roman abacus. The abacus, which used Roman numerals, was ideally suited to the counting of Roman currency and tallying of Roman measures. Negotiatores, mercatores and pedlars The negotiatores were in part bankers because they lent money on interest. They also bought and sold staples in bulk or did commerce in wholesale quantities of goods. The argentarii acted as agents in public or private auctions, kept deposits of money for individuals, cashed cheques (prescriptiones) and served as moneychangers. In some instances the argentarii are considered a subset of the negotiatores and in others as a group apart. The argentarii sometimes did the same kind of work as the mensarii, who were public bankers appointed by the state. They kept strict books, called tabulae, which were treated as legal proof by the courts. The mercatores were usually plebeians or freedmen. They were present in all the open-air markets or covered shops, manning stalls or hawking goods by the side of the road. They were also present near Roman military camps during campaigns. They sold food and clothing to the soldiers and paid cash for any booty coming from military activities. There is