From: Colin Forrester 
Newsgroups: uk.finance
Subject: Re: Credit Card Security Digits
Date: Tue, 11 Apr 2006 14:08:29 +0100

Ronald Raygun wrote:

>> The final 3 characters appearing on the reverse of the card. What is the
>> purpose of these?
> 
> They are to prove that the actual card is in the shopper's possession.
> The point is that these figures are never printed on transaction
> receipts, and therefore the genre of fraud eliminated is that of
> people rummaging through bins for such receipts.

This may have been the original reason but since the full card number 
isn't generally printed on receipts now (here or abroad), the ability to 
determine a card number from bins is much reduced.  The three digits (4 
for Amex cards, conveniently printed on the front of the card for 
photocopying!) are now routinely requested by all and sundry - so as to 
make the system much less secure.