Many travelers should not be much affected by the new carry-on luggage rules adopted this year by U.S. airlines. Though the rules were required by the Federal Aviation Administration as a safety measure, they tend to be sufficiently permissive to allow for about as much luggage as an average passenger has been accustomed to toting aboard.

The travelers whose habits may be altered are those who prefer to keep everything with them -- perhaps fearing their luggage might be lost if checked -- vacationers taking along large gift packages to friends and relatives, and shoppers who have an armful or two of souvenirs they are carrying home.

The airlines say they are serious about enforcing the luggage restrictions, and Eastern Airlines spokeswoman Virginia Sanchez warns that "if some passengers are obstinate" and insist on carrying aboard excessive luggage, "it will only delay the flight" until the luggage is checked.

American Airlines spokesman Jim Brown says American has established special tags and procedures to check excessive baggage at the boarding gate. So far, "it's been a very smooth process. There have been very few glitches."

Continental's Rick Scott sees the new rules as a way airlines can more equitably distribute storage space, so that even last-minute boarders have a chance at an overhead bin. With each passenger limited to two bags, he says, "the first guy on doesn't get everything."

"The airlines seem to be sticking to it," says Pam Zirkle of the Cruise Company of Georgetown. Like other travel agents, she warns clients about the new rules. Her big concern is cruise passengers shopping before the return flight home. "They go to Nassau and buy 200 straw hats." She's exaggerating, of course, but her point is clear. Travelers now have to pay attention to how much they are carrying aboard.

The Association of Flight Attendants, whose members have long advocated carry-on luggage limits, says it has noted "modest improvements" in the reduction of excessive luggage since the rules took effect Jan. 1. The organization expects to see even greater improvements aspassengers and airline personnel become more familiar with the rules, according to spokesman Matthew Finucane.

Under the new rules, each passenger now generally is permitted to carry aboard two medium-sized suitcases (one of which may be a hanging bag) as well as certain personal items such as a purse, overcoat, camera, umbrella, reading material and infant bag. However, all carry-on luggage must fit within certain size limitations. And, as has been standard practice all along, all luggage must fit either in an overhead bin, in a garment rack or under the seat.

The most controversial point in the new rules, perhaps, is the decision by most airlines to exclude briefcases as personal items. If you carry one, it counts as one of the two pieces of luggage you are permitted to carry aboard, as opposed to a purse, which is considered a personal item rather than a piece of luggage. The argument can be raised that business travelers should at least have the choice of carrying either a purse or a briefcase as a personal item.

Each airline has adopted its own set of rules, although the rules are based on guidelines suggested by the Air Transport Association, which represents the major U.S. carriers. As a result, the rules vary somewhat from one airline to another. Eastern Airlines, for example, currently limits passengers to two pieces of carry-on luggage with the combined dimensions -- height, width and length -- of 45 inches. Other airlines allow one under-the-seat bag with combined dimensions of 45 inches -- plus either a second bag for the overhead bin with combined dimensions of 60 inches or a hanging bag with combined dimensions of 72 inches.

In effect, Eastern is limiting its passengers to considerably less carry-on luggage than other airlines, a policy that spokeswoman Sanchez says is under study.

There are other variables. Some airlines, among them Pan Am, say they will allow more carry-on luggage if a flight is only partially full. The amount of luggage you can carry aboard also depends on the type of aircraft you are flying. Some wide-bodied planes have larger overhead bins, permitting larger luggage and more of it. On a small commuter aircraft, you may be allowed only one piece of luggage, or none at all.

The obvious difficulty for passengers is to know in advance when a plane is going to be empty enough -- or is equipped with the larger bins -- to permit them to take aboard more than two pieces of luggage. Airlines such as United prefer to downplay these possibilities, aware that some passengers may take the chance that extra space is available. If it isn't, the luggage must be checked at the gate, which may delay the flight.

Anyone who has flown in recent years is aware that excessive carry-on luggage had become an irritating problem.

As a passenger who checked all of your luggage, you could board your plane expecting: a) to be bopped in the head by someone struggling down the aisle with one too many suitcases, b) to find the bin above your seat already filled, so there was no convenient place to put your coat, c) to be held up leaving the plane by someone trying to extricate a large suitcase from a small bin.

The Association of Flight Attendants raised a safety issue. Attendants and passengers were in danger of injury from heavy items falling from overstuffed bins. Such injuries were a possibility not only during a turbulent flight, says Finucane, who is the association's air safety and health director, but any time someone opened a full bin.

The Federal Aviation Administration has long required that any carry-on luggage be properly stowed before takeoff. But, says spokesman John Leyden, "the airlines hadn't been doing a good job of enforcing" that rule. Improperly stowed luggage could hinder escape in case of an accident.

On the other hand, the traveling public had heard any number of horror tales about lost or delayed luggage spoiling a holiday or a business trip. To prevent such an occurrence, many travelers preferred to keep their luggage with them. Others saw it as a way to avoid long waits at the baggage claim area.

In the face of a disintegrating situation, the FAA ordered the airlines to come up with a written plan for handling carry-on luggage. The plans, put into effect Jan. 1, all have been submitted to the FAA and approved, according to Leyden. One immediate benefit of a written plan was to give flight attendants specific regulations to point to when confronted by a passenger with too much luggage or oversize pieces. "It takes the monkey off the attendant's back," says Leyden.

The FAA also required airlines to appoint someone outside the aircraft to screen carry-on luggage before a passenger boards and to select an on-board crew member to make sure all hand-carried items are securely stored.

To meet the FAA's requirement for a written plan, the Air Transport Association drafted its model guidelines with the help of the airlines themselves. Each airline has adopted the model with variations.

Among the provisions of the model, as outlined by association spokesman Tim Neal:

Carry-on luggage is limited to two bags, although more or fewer bags may be allowed depending on how full a flight is.

Excluded from the limit are purses, overcoats, umbrellas, cameras (but not camera bags), a reasonable amount of reading material and infant or diaper bags.

Each piece of carry-on luggage should fall within the following size limitations. To fit under a seat: no more than 9 inches wide, 14 inches tall and 22 inches long. To fit in an overhead bin: 10 by 14 by 36. To fit in a garment rack or closet: 4 by 23 by 45. Larger luggage may be permitted depending on how full a flight is or if the plane is equipped with larger storage bins.

To alert passengers to the new rules, United has printed its baggage specifications on its ticket envelope. Like other airlines, it also makes frequent announcements over the airport public address system. Some airlines are spelling out the details in their in-flight magazines.

As a result of the carry-on restrictions, anyone planning a trip that involves a flight should consider the following:

Travel light. It is a guiding maxim of many experienced travelers, especially if you are on a trip that takes you to several destinations. By trimming your wardrobe, you may be able to fit everything into the two allotted suitcases.

Carry aboard the essentials. Though you may have to check everything else, keep with you medicines, eyeglasses, important documents. If possible, pack in your carry-on luggage at least enough clothing to get you through the first day or two of your trip. For example, if you're headed for a beach resort, pack a swim suit and light evening garments. If you have a business meeting, pack the proper attire.

If you are a shopper, get the stores -- at least those in the United States -- to mail home as many of your purchases as possible. Or check all your clothing and limit your carry-on luggage to your souvenirs.

If you are in doubt about whether your suitcase qualifies as carry-on luggage, consult the airline you are flying.