In the thirteenth century, Florence, Genoa, and Venice were the chief monied centres. In these cities the purchase and sale of commercial paper was, at the outset, monopolized by a body of money-changers, who, in Venice at least, seem to have been controlled by the council of merchants, and who probably were not always in the best credit. At all events, they were required in 1318 to make a deposit of £3,000 as security for their customers, and afterward the amount was increased.[163] Possibly some such system of deposits may have originally formed the capital of the Bank of Venice, but everything relating to the organization of the mediæval banks is obscure. All that seems certain is, that business was conducted by establishments of this character long before the date of any records which now remain. Amidst the multiplicity of mediæval jurisdictions, not only did the currency become involved in inextricable confusion, but it generally was debased through abrasion and clipping. Before clearings could be conveniently made, therefore, a coinage of recognized value had to be provided, and this the banks undertook to supply by their system of deposits. They received coin fresh from the mints, for which they gave credits, and these credits or notes were negotiable, and were always to be bought in the market. The deposits themselves were seldom withdrawn, as they bore a premium over common currency, which they lost when put in circulation, and they were accordingly only transferred on the books of the corporations, to correspond with the sales of the notes which represented them. Thus merchants from all parts of Europe and the Levant could draw on Venice or Genoa, and have their balances settled by transfers of deposits at the banks, without the intervention of coin. A calculation has been made that, by this means, the effective power of the currency was multiplied tenfold. Of all these institutions, the corporations of Genoa and Venice were the most famous. The Bank of Saint George, at Genoa, was formally organized in 1407, but it undoubtedly had conducted business from the beginning of the twelfth century;[164] next to nothing is known of the development at Venice. Probably, however, Florence was more purely a monied centre than either Venice or Genoa, and no money-lenders of the Middle Ages ever equalled the great Florentine banking families. Most of the important commercial centres came to have institutions of the kind. The introduction of credit had the same effect as a large addition to the stock of bullion, and, as gold and silver grew more plentiful, their relative value fell, and a general reform of the currency took place. Venice began the movement with the grosso, it spread through Italy and into France, and the coin of Saint Louis was long considered as perfect money.