An hypothetical quantity of fine silver, employed as a money-valuer in the old Bank at Hamburg, and used by the Hanseatic League. Deposits in gold and silver coins were credited in Marco Banco, and all banking accounts were carried on in Marco Banco. The benefit was this: Marco Banco was invariable, but exchange varies every hour. The bank not only credited deposits by this unvarying standard, but paid withdrawals in the same way; so that it was a matter of no moment how exchange varied. I put 1,000 into the bank; the money is not entered to my credit as 1,000, but so much Marco Banco. The same process was adopted on withdrawals also.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894