How good is the guarantee on a guaranteed hotel room reservation? If you think it assures that a room in the hotel of your choice will be waiting, you could be in for a rude surprise. On the other hand, it's unlikely you will end up sleeping on a park bench.

Like airlines, hotels sometimes overbook, taking reservations for more rooms than they have available. It's a way of compensating for last-minute cancellations or no shows. If everyone shows up, however, some people may not get a room. Airlines call it getting "bumped." In hotel jargon, it's getting "walked."

It almost happened recently to two business travelers from Washington who were checking into a big downtown Chicago hotel to attend a trade show. They had reserved a room well in advance and had given the phone reservation clerk a corporate credit card number to guarantee the reservation. A common practice, this meant that even if they didn't show up at all the hotel could charge them for one night's stay. Supposedly, a room would be kept available for them throughout the night.

As it turned out, their plane was delayed, and they didn't arrive at the hotel until about 8:15 p.m. Tired and hungry, they were dismayed to hear the desk clerk inform them that just two minutes earlier, the hotel had filled its last empty room. But because they had a guaranteed reservation, the clerk said she would "walk" them to a room in another hotel, pay the cost of a night's lodging and take care of the cab fare to get there and then back again the next day. (Beware of the term "walk," which may actually mean a lengthy cab ride.)

In the highly competitive hotel industry, the offer of a free room in a comparable hotel and cab fare has become standard practice -- at least among the better establishments -- when your hotel cannot honor a guaranteed reservation. Unlike the airlines, which by federal regulation must provide compensation to bumped passengers, hotels generally are under no legal obligation to assist walked guests. However, to avoid earning a bad name in the lodging industry, they make the effort to find a substitute room.

Marriott and Hyatt, for example, are among the large chains that have made this a corporate policy. They also give you a free phone call home to let family, friends or business associates know of your altered lodging. At the very least, a hotel should find you alternate lodging and pay the cab fare to get you there. There's one more development in the tale of the two Washington travelers. They might have taken the hotel up on its offer. However, on that night Chicago apparently was bustling with conventions, and the nearest vacant room that could be found for them was a time-consuming ride away in the distant suburbs. The pair put up a small fuss, and the hotel managed to come up with one room for them to share on the premises -- a twin-bedded, one-room suite that was ordinarily used as a small conference facility. The next night they got the rooms they had reserved and a basket of fruit and wine as an apology.

Their story illustrates what other frequent travelers have discovered. A guaranteed reservation is not absolute, and you may some day find yourself getting walked -- perhaps farther than you might reasonably agree to. However, there are some measures you can take to try to avoid getting walked, and it helps to be aware of what many hotels will do to compensate you for the inconvenience.

"I'd be lying if I said we didn't overbook," says Steve Weisz, vice president of room operations and related services for Marriott Hotels and Resorts, "but we try to be careful."

From the point of view of the hotel, the practice is necessary. "Lots of people don't bother to call and cancel," says Chris Thompson, reservations manager for the Sheraton-Plaza Hotel in Chicago.

How often are hotel guests walked? "Infrequently," say Weisz and spokesmen for other hotel chains, but that's probably not much consolation to the would-be guest whose reservation is not honored.

"Not even we can guarantee it won't happen," says Doris Davidoff of Belair Travel in Bowie, a nationally recognized authority in travel agency management. "The really experienced frequent traveler knows it can happen. They understand it's a fact of life. They accept it."

Unlike airlines, says Davidoff, which can ask for volunteers when it has to bump passengers, "Once people are in their rooms, the hotel can't broadcast over a public address system that 'We're in an overbooking situation.' "

For business travelers, the worst problems occur when a city plays host to a large convention or two or three smaller ones that have filled up most of the good downtown hotels. For vacationers, the problem at resorts can be overbooking on a busy three-day holiday weekend. If you know either is a situation you are headed for, you may want to consider planning to arrive by early afternoon rather than in the evening.

Tightly booked hotels also end up with too many guests when someone who has promised to leave decides to stick around for another day or two. One Washington association executive showed up for her national convention in Minneapolis last year expecting a nice suite only to find the head of a just-ended convention still occupying it. She moved in the next day. Hotel spokesmen say they don't forcibly evict stayovers.

Occasionally, the villain is a hotel chain's toll-free reservation service, says the Sheraton-Plaza's Thompson. For whatever reason, the computer operation keeps booking new guests even though the hotel is filled. What this means is that "somebody along the line screwed up." One night when the hotel was overbooked, he had to make 30 phone calls to find alternative rooms.

Some types of reservations, it seems, are better than others. Weisz points out that Marriott classifies reservations into three categories. The one with which you have the best chance of getting a room in an overbooking situation is a reservation with an advance deposit. "We jump through hoops to honor advance deposits." Next in importance is a reservation guaranteed with a credit card. At the bottom is the "time" reservation. This is one that is held until a specified hour -- usually 6 p.m. but sometimes earlier. If the guest doesn't arrive, the room is given up.

While a prepaid reservation should give you added clout with a desk clerk, it can also be of help to phone ahead if you are running late, says Weisz. In hotel terminology, it's "a call to confirm. We do all that's in our power to recognize it."

Travelers who do find themselves getting walked would do well to inform the offending hotel what other chains, such as Marriott and Hyatt, provide as compensation. It is a strong bargaining chip when the hotel is clearly at fault.

According to spokeswoman Carrie Reckert, Hyatt will find a room for you in a comparable hotel that is close by, if possible. (Many hotels will begin lining up alternate rooms when they see an overbooking situation developing.) The Hyatt will do the phoning.

Hyatt pays for the substitute hotel room and for the cab fare there and back again. It also allows one free long-distance phone call to notify family or associates. The Hyatt switchboard is alerted to the change so calls and messages can be forwarded to the new hotel. Walked guests go to the top of the list for a Hyatt room the next day.

When they do check back into the Hyatt, says Reckert, they are given "a special amenity," such as a basket of fruit or a bottle of wine, "and a form of apology" -- perhaps a phone call from the Hyatt manager. "It's to let them know we're sorry this happened."

If a travel agency has handled your reservations, says Davidoff of Belair Travel, it may be able to get you some compensation if the offending hotel fails to satisfy you. However, if you have been treated in "a reasonably expeditious manner" -- awarded a comparable room and cab fare -- "then there's not much an agent can do except sympathize."

One thing to keep in mind is that not every hotel may, indeed, have a spare room held back for emergencies, and no amount of heated argument from you will produce one. According to Thompson, at least, the two Washington travelers were lucky. "People believe we're hiding rooms, but it's just not the case," he says. "If I've got a room, I want the revenue for it."

BRITAIN FOR SENIORS: A variety of discounts awaits travelers to Great Britain who are age 60 and older. The bargains are detailed in a new 96-page guide, "Britain for the Very Good Years," available at no charge from the British Tourist Authority.

For under $10 (6 British pounds), for example, seniors can purchase an English Heritage Senior Citizens Pass, which provides admission to 350 historic sites in England, among them Stonehenge and Hampton Court Palace. Other sites in Scotland and Wales honor the pass with half-price admission.

Older travelers who purchase a BritRail Pass can enjoy Britain's trains in first-class coaches for only a little bit more than they would have to pay for economy class. (BritRail passes, which are sold only outside Great Britain, can be purchased from U.S. travel agents.)

The guide also describes discounts for bus travel, lodgings, meals, theatrical performances, concerts and tour packages.

For a copy, contact: British Tourist Authority, 40 West 57th St., New York, N.Y. 10019, (212) 581-4700.

BAREBONES ON THE CARIBBEAN: Barebones sailing is for the experts, but a local yachting organization that is sponsoring a barebones cruise of the British Virgin Islands during Thanksgiving Week has opened the trip to beginners also.

Going "barebones" means chartering a small yacht for private use without an accompanying crew. On this outing, you and experienced members of the organization, Chesapeake Bay Mates Inc. of Annapolis, serve as crew and captain. Up to 10 vessels, each accommodating an average of eight participants, will cruise the Caribbean together for seven days.

Chesapeake Bay Mates is a year-old nautical referral service that matches yacht captains with potential crew members, ranging from skilled racing enthusiasts to novices interested in gaining sailing skills. It works this way:

A graduate of a local sailing school may want practical experience before acquiring a boat. A captain may need an extra hand for a day's outing or a week's cruise up or down the coast. Both parties list themselves with Chesapeake Bay Mates for a $35 per person annual fee (or $50 per couple). About 200 captains and 500 potential crew members are now on Chesapeake Bay Mate's roster, according to spokesman Michael Mullins.

About half the berths on the Virgin Islands cruise will go to organization members, and each ship will have an experienced captain. The rest of the berths are open to nonmembers.

Departure is Saturday, Nov. 21, from Baltimore-Washington International, and the cruise gets underway the following morning from Tortola. The cruise -- a new island each day -- concludes the following Sunday. The cost is $1,095 per person and includes air fare, airport transfers in Tortola, meals aboard ship and three on-shore parties.

For information: Chesapeake Bay Mates, 3 Church Circle, Suite 240, Annapolis, Md. 21401, (301) 267-7342. The direct line from Washington is 858-6221.

ART FOR THE AMATEUR: With sketchbook in hand, travel the Soviet Union for two weeks this September with a noted Washington-area artist, Samuel Bookatz. Designed for amateur artists, the tour will visit Leningrad, Moscow and Yalta as well as Helsinki. ,

The tour will provide opportunities to watch master Russian craftsmen and artists at work, and the itinerary includes visits to major Russian art collections. Meanwhile, you can sketch or paint scenes of Soviet life with advice from Bookatz.

Departure is Sept. 9. The price is $2,499 per person (double occupancy), which includes round-trip air fare from New York on Finnair, lodging for 15 nights and all meals in the Soviet Union.

For information: Tour Designs Inc., 510 H St. SW, Washington, D.C. 20024, 554-5820.

BUSINESS BRIEF: Pay phones recently have made their appearance on transcontinental flights, and now they are being placed on cruise ships. At least 10 cruise lines have introduced them in the past year, and more are expected to do so soon.

Though a cruise is supposed to mean relaxation, you will be able to keep in contact with your office or stockbroker by direct dial from any sea the ship is sailing. Via long distance, you also can tap the line of credit on your Visa, MasterCard or Choice credit cards for ready cash at sea -- perhaps to pay your losses in the ship's casino.

On some phones, you can punch a button and call up stock market quotations on a small screen or punch another button to make a rental car reservation. Other possibilities include teleconference calls, facsimile transmissions and a link up with the computer in the home office.

The idea is to provide cruise passengers with office facilities at sea, says Robert Eichberg, vice president of marketing for COMSAT'S Maritime Services division.

Among the participating lines are Cunard, which introduced the pay phones on the newly refurbished Queen Elizabeth 2; Sitmar; Holland America Line and Norwegian Caribbean Lines.

The credit card pay phones are being installed by The Communications Satellite Corporation (COMSAT). According to COMSAT, the phones make use of a special system of maritime satellites that previously had been used mostly by cargo ships, offshore oil rigs and expensive yachts.

In the past, passengers who wanted to phone from ship to shore did so over an open radio frequency. Often there was a lengthy wait before the call could be completed. And other radios could easily receive the message. The satellite calls are "confidential," says COMSAT.

The basic charge to phone from a ship sailing anywhere in the Atlantic or Pacific oceans to the United States is $5 for 30 seconds, although individual cruise lines may add a surcharge. There are additional charges for phoning from ships in the Indian Ocean and for placing calls to countries other than the United States. Calls can also be made from shore to ship at the same basic rate.

For information: COMSAT Maritime Services, Marketing Department, 22300 Comsat Dr., Clarksburg, Md. 20871, (800) 424-9152.