system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: The back of my debit card says, "Not valid unless signed". When would this ever matter?
gpt
The use of these cards is partially governed by the agreements that the credit card transaction networks have with merchants. (Since your debit card probably has the option of using Visa, Mastercard or American Express for payment.)

 Under those agreements, a merchant isn't required to accept an unsigned card, and it's recommended to check an ID of the person presenting it. By contrast, merchants should not require IDs of people presenting a signed card, and just compare the signature on the back to that on the receipt if desired. This is a courtesy to the cardholder.