"For just $1,899, the world is on us," beckons Trans World Airlines in its newest promotion for special around-the-world fares. Like similar globe-circling air fares offered by other international carriers, it's a very good bargain -- but only if you know exactly what you are buying.

Don't be misled, for example, by the airlines' claims that you can choose from dozens of destinations. Very strict routing restrictions on these tickets -- no matter what the airline -- make it impossible to visit all the cities the carriers service. Before you even get off the ground -- literally -- you may have to eliminate from your itinerary some of the places you had hoped to visit.

On the other hand, these fares make it convenient -- and relatively inexpensive -- to see several major cities on two, three or four continents that you might not otherwise travel to in a lifetime of one-at-a-time round trips. Depending on the airline, east-bound ticket-holders from the mainland United States can plan stops, say, in Paris, Cairo, Bombay, Singapore, Hong Kong, Tokyo and Honolulu -- among many other possibilities.

The tickets are designed for people who have dreamt of some day actually circling the globe -- it's a milestone in any traveler's life -- and who want to make frequent stopovers en route. The principal benefit of the tickets is the unlimited stopovers that are permitted. If you bought a regular ticket, paying for multiple fares point to point, you would spend considerably more, say the airlines.

The fares would not be of much value to you if you are interested only in visiting one or two places. TWA currently is quoting a round-trip fare between Washington and Bombay of $1,122, which is substantially less than the around-the-world fare. The fare permits only one stopover each way to and from Bombay.

Time is another consideration. Travelers should give themselves at least three weeks to a month or more to make a circumnavigation, if only to avoid constant jet lag. However, tickets are valid for at least a half-year's travel, and some are good for a full year. Ideally, the around-the-world air fares are best used when you have time to linger along the way. Too hurried a trip, and your memory of places blurs.

TWA has found that the tickets are bought by a wide variety of travelers, says Sherry Lewis, supervisor of interline sales and marketing. Among them are young backpackers on a tight budget, older couples who have retired and even some business travelers who use the tickets to visit clients in several countries.

Travelers on limited time might stick to the cities on their route -- a quick tour of Cairo, for example, before moving on to Bombay and Tokyo. Those with more leisure might explore Egypt more thoroughly, returning to Cairo when they are ready to fly on to another country. Some more adventurous travelers might fly into one country and then continue overland by bus or train into a neighboring country before they resume their flight. The cost of overland transportation is additional.

Here's how the special around-the-world fares work:

None of the world's airlines has a flight that completely circles the globe. So airlines with an around-the-world fare have linked with one or more other airlines to provide a complete circumnavigation. TWA is associated with Singapore Airlines; Japan Air Lines; Korean Air; Qantas, the Australian airline; and Cathay Pacific Airways, the Hong Kong airline. Pan American and Northwest, two other U.S. carriers also offering globe-circling fares, are similarly linked with these and other airlines.

In most cases, travelers are allowed to fly only on the originating air carrier -- TWA, Pan Am or Northwest, for example -- and one other airline. So you must pick a combination of two airlines that gets you to most of the places you want to see.

The U.S. carriers, of course, will carry you across the Atlantic Ocean. Thereafter, the routing gets a bit tricky. If you have a big interest in Malaysia and Indonesia, you might choose the routing onward to Asia provided by TWA and Singapore Airlines, because Singapore has many flights in that region of the world. If Australia is a must, you will want a combination that includes an Australian-bound airline such as Qantas.

TWA's $1,899 fare, one of the lowest available, is valid only on routes served by the Singapore, Japanese and Korean carriers. It is good for a seat in economy class. A TWA ticket via Australia on Qantas is $2,599, because of the additional air miles to get there. Other airlines offer fares that are competitive. You may want to pay a slightly higher fare, however, for an airline combination with routing that better suits your plans.

Before you plot your route, however, it is important to understand the restrictions and potential drawbacks of these global tickets:

They are not passes permitting you to fly anywhere you want. Generally you must travel in an eastward or westward direction. You may not backtrack -- you can't fly from London to Paris and back to London again, for example. Most routings from the United States take you through Europe and the Middle East to India, the North Pacific and Japan. A few head from Europe to Australia and the South Pacific islands. British Airways and Northwest have routes through Africa.

You must choose your routing and your stopovers before you buy the ticket. You usually are required to reserve your first flight, but you can leave departure dates open for the rest of the trip. This gives you scheduling flexibility should you decide to remain in a city longer than you had planned. Most airlines allow you to make one free change in your itinerary. Afterwards, a charge is made for each change, usually $25.

Consumer Reports Travel Letter, a monthly publication of Consumers Union, made a study of around-the-world fare offers last year and pointed up two other potential problems to consider when plotting an itinerary.

First, airlines don't fly to all their destinations in an orderly sequence. A westbound carrier from Los Angeles across the Pacific might stop at certain airports on one day and different airports on another day. You may be faced with choosing between two places you want to visit because your carrier does not go to both of them on the same flight.

As a practical matter, this means you shouldn't take at face value an airline map dotted with the names of the places it serves. You also have to check the airline's timetable (and that of any other airline it is associated with) to determine how many you actually can get to on a continuous eastbound or westbound loop.

Second, an airline many serve some destinations as infrequently as once a week. Check the timetables also for flight frequency if you don't want to spend a week waiting for the next onward plane.

To plan a trip, obtain a destination map and timetable from one or more international airlines offering an around-the-world fare. Pick the places you want to visit, and then work with an airline representative or a travel agent to include as many of them as possible on your ticket.

As to whether you should head east or west, Consumer Reports makes this observation: On eastbound flights, you travel the long-distance hauls overnight, saving days for sightseeing. On westbound flights, most of the long stretches are during the daylight, which means you won't have to spend as many nights in flight trying to sleep sitting up.

Among the around-the-world fares:

TWA: A TWA ticket is $1,899 economy and $3,699 first class on routes shared with Singapore Airlines, Japan Air Lines and Korean Air; $2,099 economy and $3,999 first class on routes shared with Cathay Pacific; and $2,599 economy and $5,399 first class on routes shared with Qantas. With Japan Air Lines, the ticket is good for six months. With the other airlines, it is good for a year.

Pan American: Pan Am's economy fare is $2,099 and is good for a full year. Pan Am is linked with Cathay Pacific in the Orient and United Airlines across the Pacific. First-class fare is $3,999. A route via Australia with Qantas is $2,599 economy, $4,150 business class and $5,399 first class.

Northwest: Northwest is linked with more than a dozen other airlines, but you can only choose one from among them. The economy fare on most routes is $2,099, and a ticket -- again on most routes -- is good for one year. First class is $3,799. Via South Africa, the fare is $2,677 economy and $4,016 first class. On some routes, a ticket is good for only six months.

Among the airlines associated with Northwest are Air France, Air India, Cathay Pacific, Garuda Indonesian Airways, Gulf Air, KLM Royal Dutch Airlines, Kuwait Airways, Malaysian Airlines System, Pakistan International, South African Airways, Sabena-Belgian Airlines, Saudi Arabian Airlines and Thai Airways International.

LITERARY LONDON: Where do you begin, in the midst of a wealth of possibilities, to explore London's literary heritage?

Plantagenet Tours, a London-based tour organizer specializing in historical tours of Europe, has put together a nine-day package that explores the lives and haunts of Samuel Johnson, John Keats, Charles Dickens, Thomas Carlyle, Oscar Wilde, Virginia Woolf and George Orwell, among others.

At a relaxed pace, tour participants visit the writers' former residences (where possible), restaurants where they dined and London sites they wrote about. Day 4 on the itinerary, for example, features a look at Dickens' house on Doughty Street in Bloomsbury, and then the group will follow a route that takes in places associated with "Bleak House," "Great Expectations" and other stories.

Departure is Nov. 28 from New York. The price is $1,585 per person (double occupancy), which includes round-trip air fare between New York and London, seven nights' lodging at the London Embassy Hotel and breakfasts and dinners.

For information: The Plantagenet Tours, 85 The Grove, Moordown, Bournemouth, Dorset, BH9 2TY, England, (800) 521-4556.