Your Reservation Absolutely Assures You a Room, Sort Of : Lodging: Overbooked hotels reserve the right to ‘walk’ you elsewhere for the night, at their expense. Check before you go.

What does it mean when a hotel offers you a chance to “guarantee” or “assure” your room by giving your credit card number in advance? Most travelers presume it means a room in the hotel will be held in their names, no matter what. After all, the vast majority of reservations do work out that way.

But not all. Overrun on a convention week or otherwise caught off guard, most hotels overbook now and then, and end up telling guests that their “guaranteed” rooms have been given to someone else. To placate guests at such times, industry insiders say, most hotels have settled on a common make-good strategy: Find the inconvenienced guest a room elsewhere, and cover the cost of his first night.

Unfortunately, as my friend Steve discovered last month, a traveler without that specialized knowledge runs the risk of getting the short end of the stick.

Cut now to the Hotel Triton in San Francisco, on the evening of Sept. 22. Facing a one-night business trip to the city and having heard good things about the playful design and convenient location of the Triton, Steve had booked a room there through a local reservation agency.

The idea behind guaranteeing a room is appealing: You give your credit card number to assure payment whether you show up that night or not, and in exchange you have the freedom to arrive any time after the lodging’s usual 4 p.m. or 6 p.m. check-in hour.

If you reserve a room without using a credit card number and don’t show up by early evening, the hotel understandably feels free to give the room to someone else.

Travel agents say virtually all business travelers and most leisure travelers give their credit card numbers in advance these days.

In Steve’s case, the arrangement was made more than a week in advance; he gave his American Express number; he received paperwork and a confirmation number in the mail. But when he got to the hotel counter, the clerk told him there was no room for him.

What, asked Steve, about his confirmation number and his piece of paper? No room.

Had his reservation been lost? No. There was just no room.

The clerk eventually explained that another guest had decided to extend his stay, thus occupying the room that would have been Steve’s. Then the clerk told him a fall-back room had been reserved for him a few blocks away at the Sir Francis Drake --a comparably priced property run by the same management as the Triton, the Kimpton Group, but a much less interesting place. For his trouble, the Drake gave Steve a free night’s parking. Value: $23.

“Why,” asked Steve, “should I have to pay for someone else’s decision to stay another night?”

In the hotel trade, this is known as “walking” a guest. Hotel officials say the process is rare (so rare, several spokespersons claimed, that they have no statistics on how often it happens) and undesirable. But it’s common enough, especially during big conventions, to have a universally recognized nickname, and to be specifically provided for in the policies of major credit cards and hotels.

Kenneth F. Hine, president and chief executive officer of the Washington-based American Hotel & Motel Assn., said that “if a credit card is used to guarantee a reservation, then guests have every reason to expect that there will be a room waiting for them when they arrive.” If there isn’t, Hine says, the “generally prevalent industry practice” is that the hotel pays for the traveler’s first night in comparable alternative accommodations, and also for transportation to the new property and a phone call to family or business to pass word of the unexpected change in lodgings. (There are minor variations from chain to chain; Hilton’s “dishonored reservations” policy, for instance, allows as many phone calls as are necessary to alert family and business associates of the change in address.) That practice “has evolved company by company,” Hine said, and dates back at least 20 years.

That informal custom runs essentially parallel with American Express’ “assured reservations” program, which company officials say is more than a decade old and includes a substantial majority of the major hotels in the United States. That program, officials said, requires member hotels to offer a free night’s lodging, transportation and phone call when guests with AmEx-reserved rooms are forced into another hotel.

Visa has a program for card-holders, too, but doesn’t mandate a free night’s stay.

If you want to be clear where a hotel stands, it’s probably best to ask the question directly in advance: What happens if you have to walk me to another hotel?

So what, then, is the Triton policy? Kimpton Group spokeswoman Merideth Post reports that all 13 Kimpton hotels in San Francisco do in fact have a policy of giving a free night to guests who are walked to alternative hotels. Unfortunately, she said, Steve encountered a clerk who misunderstood the policy. Now the clerk understands, Post said, and Steve has a free hotel night waiting the next time he’s in San Francisco.