I know that promissory notes have been around since the 14th century in Italy. And I know that there are a whole load of rules about the modern financial instrument as to what information it has to contain to be a proper promissory. Do we know what text was on the early forms of these notes?
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If you are looking for primary source documents from the middle ages, the Internet Medieval Source Book at Fordham University has a lot of material, usually in English translation. For example, the text of a promissory note from 1199 Messina.