In message , at 12:28:11 on Mon, 19 Mar
2012, Stephen Sprunk remarked:
What I agree we didn't get to the bottom of is how the card companies
can issue 'normal' VISA debit cards while also promising the cardholder
that they (the cardholder) will not be allowed to overdraw.

A debit card works no differently, from the network's or merchant's
perspective, from a credit card. The merchant requests authorization
for a particular amount, and the issuing bank either accepts or declines
the transaction. If the merchant posts a transaction without
authorization, it may be declined and the merchant may not get paid.

Either card type may be declined if the issuing bank sees the customer
has insufficient funds, whether such funds are drawn from a debit
account or a credit account.

sigh but we don't (here in the UK anyway) live in a world where every
transaction is authorised.
--
Roland Perry