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Paying for gas at the pump is a great convenience, but when your credit or debit card is declined, it can turn into a really frustrating experience--especially when you know you have enough money on your credit limit or in your bank account to cover the amount of gas you want to put in your car.

Cards get declined in situations like this because they can't tap into an amount of money large enough to cover what's called an authorization. When a card is authorized, the issuers of the card--which is the bank, not a payments network like Visa or MasterCard- -take an amount of money that will generally cover the anticipated transaction and then some and set it aside to use specifically for that purpose. Once you finish the transaction and the gas station closes it out, any leftover monies return to your account.

The amount held in authorization for a gas station transaction varies from bank to bank. Some banks only withhold a few dollars; others withhold up to $75. "Essentially they put a hold on the card for a certain amount of money," says Ben Jackson, senior analyst at Mercator Advisory Group. "They don't know how much the total bill will be, so they preauthorize for a certain amount so they know it will be covered."

For consumers, this means that you must have enough available funds to cover that withheld amount. Say you have $50 open on your credit limit, and you want to put $25 of gas in your car. However, your bank may make a $75 authorization hold on transactions at gas stations. Because you don't have that much credit left, the bank will decline your card.

While it may not sound fair to the consumer, this is an extremely common practice that protects retailers against potential fraud. "Transaction limits may vary throughout the U.S. and are subject to change based on a number of factors including card issuer authorization limits, system limitations, and Brand Association guidelines designed to mitigate potential fraud risks," says Kayla Macke, media relations and Issues and Crisis Management coordinator for Shell Oil Company, in an e-mail statement.

Gas stations aren't the only places where you run into card authorizations. This practice is also common at restaurants, bars and hotels.

Charles Badendieck, general manager of the Hampton Inn in Woburn, Mass., has had his fair share of those conversations. He explains that when guests check into a hotel, the hotel generally authorizes a charge good for the length of the stay, plus 10 to 15 percent extra to cover taxes and incidentals the guest may charge to the room. While the card used to authorize these charges isn't actually charged until check out, the guest isn't able to use that money that is being held.

Badendieck says this can really become an issue for a guest if he uses a debit card. If the authorization amount is $200, the act of the authorization causes that dollar amount to be held. "It really holds $200 of your account," says Badendieck.

When considering hotel charge authorizations, the amount held usually equates to your entire stay, so if you have a six-day stay, the hotel will authorize a payment for the room, taxes and incidentals. If your account can't cover this amount, you can't use the card.

What happens when you check out, and the amount of your stay is less than the authorization price? That extra amount gets put back into your account or added back to your credit limit--eventually.

"The problem is that these holds sometimes stay on the cards so that you end up with a hold that will last a couple of days until the transaction is settled," says Jackson.

This isn't the case for all authorized holds, but it varies from site to site. "It depends on the way [the company] handles and settles those transactions," says Jackson. "As long as the transaction is open, so is that full authorization."

Transactions can be open anywhere from real-time to a few hours, to a few days. It all depends on each individual retailer. Still, payments providers do have measures in place to get funds back into your account as soon as possible. "To help ensure that holds do not disrupt cardholder access to the funds in their accounts, Visa requires that Visa check card issuing financial institutions release all holds within three business days of the authorization request or when the transaction clears, whichever is earlier,” says Visa Inc. in an e-mail statement.

Badendieck says that closing out transactions in a hotel can very well take a few days. Sometimes a Friday check out doesn't get closed until after the weekend because the banks aren't open on Saturday and Sunday. If you're planning to pay with a debit card, three days could be a long time to keep your cash tied up. In those cases, "it's good to use a credit card only," says Badendieck.

With the convenience of credit, authorization holds are a way for retailers to protect themselves against fraud. Understanding how they work and learning how to account for them can help you avoid having to have some unpleasant conversations.