I was signing the receipt for my credit card purchase. The clerk noticed that I had not signed my name on the back of the credit 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 I just signed 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.