I was signing the receipt for my credit card purchase when the clerk noticed that I had not signed on the back of the card. She informed me that she could not complete the transaction unless the card was signed. When I asked why, she explained that it was necessary to compare the signature on the credit card with the signature on the receipt. So I signed the credit card in front of her. She carefully compared that signature to the one I signed on the receipt. As luck would have it, they matched