When credit cards were first introduced, merchants used mechanical rather than magnetic portable card imprinters that required carbon paper to make an imprint. They did not communicate electronically with the card issuer, and the card never left the customer's sight. The merchant had to verify transactions over a certain currency limit by telephoning the card issuer. During the 1970s in the United States, many merchants subscribed to a regularly-updated list of stolen or otherwise invalid credit card numbers. This list was commonly printed in booklet form on newsprint, in numerical order, much like a slender phone book, yet without any data aside from the list of invalid numbers. Checkout cashiers were expected to thumb through this booklet each and every time a credit card was presented for payment of any amount, prior to approving the transaction, which incurred a short delay.