Clavis commercii: or, The key of commerce [electronic resource] : Shewing, the true method of keeping merchants books, after the Italian manner of debtor and creditor; in one hundred and twenty propositions, containing most cases relating to merchandize. With a practical waste book, journal and ledger, and directions how to ballance the ledger, and transfer the ballance thereof, as an inventory into a new ledger. Designed for the help and assistance of young merchants, at their first entrance on their apprenticeship to their masters. By J.H. of London, merchant.