A trade card is a square or rectangular card that is small, but bigger than the modern visiting card, and is exchanged in social circles, that a business distributes to clients and potential customers, as a kind of business card. Trade cards first became popular at the end of the 17th century in Paris, Lyon and London. They functioned as advertising and also as maps, directing the public to the merchants' stores (no formal street address numbering system existed at the time).