system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why is there a separate security code on credit cards? If the three extra digits make it that much more secure, why not just make the number three digits longer?
gpt
In the olden days, credit cards were often not scanned with the mag strip, because the equipment was still too expensive for smaller retailers.

What they did instead was use a carbon paper and a roller machine to take an imprint of the front of the credit card with the numbers. This was commonly part of the receipt, and one copy would be torn off and given to the customer.

The problem with this, of course, is that now all these receipts you are just throwing away left and right have your whole card number on them. 

This is where the extra numbers on the back to confirm an online(or at the time, over the phone) purchase can be used,  if you only had a receipt you found with the front of someone's card, you would not have all the numbers needed to complete a transaction.