Roland Perry wrote:
at 15:39:25 on Sun, 26 Feb 2012, Adam H. Kerman remarked:

My one step: Enter PIN which is checked, and terminal asks CCC
for auth for the exact amount, checking for stolen
cards, floor limits and available credit.
Their one step: [Usually] CCC sends auth to retailer's terminal, which
displays "accepted".

Does the retailer also receive a transaction ID number, a number that
also appears on the cardholder's monthly statement?

If so, then the procedure is comparable to what happens here.

UK credit card statements (including Amex) don't usually have
transaction numbers.

Ok. I bet the transactions are numbered, though.

What I was disputing was your retailer "two pass model", with the card
itself being authorised up to some "reserved" amount, ahead of the
actual amount being claimed milliseconds later by the retailer.

The card isn't swiped twice. The merchants receives data in two steps.

I don't know why they'd do that, rather than ask for authorisation of
the actual amount first, because in the UK the amount has to be known
before you enter your PIN into the C&P device, as entering your PIN is
an agreement to pay that precise amount.

Uh, always? What do you do at a fuel station which requires a credit
card before dispensing fuel, or at a restaurant, when the tip amount
to be charged is not known until later, or at a hotel that requires
a credit card to create a reservation and the same one or another
one to check in?

The two-pass scheme is used in other circumstances, such as checking
into a hotel, when they often "reserve" an estimate of the final bill,
ahead of the day you eventually check out.

Ah. So the amount isn't always known. Do you provide the PIN
at that point, or not?

Hotels, rental car companies, may bill you for charges after you are
presented with the final bill.