The paragraph is followed by an incomplete phrase or sentence which summarizes the main idea of the paragraph. Choose the missing words of the phrase or sentence.

By the 17th century people began to leave gold with the local goldsmiths for safe keeping. Receipts of ￡1 or ￡5 were issued which could then be converted back into gold at any time. Soon these receipts began to circulate as banknotes.
The （） of banknotes.