The airline name you see on your ticket may not mean you will be flying on a plane of that line, sometimes because of a practice the airline industry calls "code sharing."

Code sharing does not signify much to the average traveler. But like people with hotel reservations who are "walked," or sent elsewhere because of overbooking, passengers quickly learn about code sharing when they end up on another airline.Take Dr. Sandra K.C. Lim and her husband, Dr. Andrew C.K. Cheng.

This spring the couple, who live on Long Island, decided to visit Mauritius, in the Indian Ocean, where Dr. Lim was born and her family still lives.

Dr. Lim left 26 years ago, and it was to be her first trip back, so Dr. Cheng urged her to buy first-class tickets. She booked first class with Lufthansa, at a round-trip cost of $6,300 a ticket.

They left Kennedy International Airport on Flight 405 on May 25. In Frankfurt they boarded the continuation of their flight, No. 6474, which turned out to be on an Air Mauritius 767 plane.

The first-class cabin was not clean, Dr. Lim said, adding that the cabin and service were "below par."

Worse, there were only two rows in first class, their row and the smoking row behind them, which meant a 12-hour flight in smoky air, to which Dr. Cheng is very sensitive.

Their travel agent did not tell them that they were changing lines, Dr. Cheng said; their travel agent said later that she did not know.

The effects of code sharing, joint operations, leases and pooling agreements are becoming increasingly widespread: in a period of recession with virtually no domestic regulation, when the European Market countries are moving toward full internal competition, almost any passenger can end up on a plane with an unexpected name and crew.

In the case of the Long Island couple, they were expecting Lufthansa service at the $6,000 level and considered that they did not get full value.

Code-sharing began in the 60s, when Allegheny Airlines began to use its code, AL, in front of the flight numbers for its connections on smaller regional airlines.

In the early 80s, other big airlines followed suit. The ticket, for example, would show Delta as the carrier all the way to Albany, Ga., but the Atlanta-to-Albany leg would actually be on a 24-passenger plane operated by Atlantic Southeast Airlines, "the Delta connection."

Opponents of this practice, which then included 12 independent regional lines, plus American and United, said it was deceptive, giving travelers the idea that they were making a connection on the same airline when they were actually changing lines.

In September 1984, a spokesman for United Airlines was quoted by the Official Airline Guide magazine Frequent Flyer as saying:

"It misrepresents two airlines with different levels of service which are indicated as one airline. We think it misleads the public. It's like buying an Oldsmobile with a Chevy engine."

But at that time, it was easier for the traveler to find out whether there was a switch of airlines.

One tipoff was that only commuter flights had four-digit numbers, so experienced travelers and travel agents would recognize that the trip would be aboard a hedge-hopper.

"Then we ran out of numbers," said Bill Jackman, a spokesman for the Air Transport Association, and the four-digit designation came into use on non-commuter flights as well.

Code-sharing was accepted by the federal regulators and the practice moved into the international field, with big lines coordinating arrival and departure times, and using each other's terminals.

Sometimes one airline flies the outbound trip and the other the inbound. Airlines in Europe were then able to advertise that they served Houston as well as New York, and domestic airlines were able to offer cities they did not fly to.

As a sales device, it was significant because two flights with a shared code may be handled in the computer as a direct flight, meaning a flight where you do not change planes although you may stop en route.

Direct flights appear on the computer screen before connections, and this means that an agent would be more likely to pick this than a connecting flight.

The practice became widespread and American and United changed their minds. United began to share codes with regional lines in 1986.

In January 1988 United worked out a code-sharing agreement with British Airways that allows British to offer flights to and from Seattle and other places in addition to Chicago, one of its ports of entry.

In fact, a Department of Transportation policy requires airlines "in any direct oral communication" to alert consumers as to what line they will fly.

The policy does not refer to any travel agent's duty on this, although Hoyte B. Decker Jr. of the department's consumer affairs division says that in this context, a travel agent is acting as an agent for the airline and shares the obligation to inform the consumer.

These days, travelers dealing with cheap-ticket offices, or even with full-service travel agencies, are almost always dealing with someone using a computer.

An overworked agent is normally reluctant to pause and call an airline to get added information on a ticket that is virtually sold. And the giveaways on shared codes, joint operations and pooled flights vary among information sources: the printed edition of the prime reference work, the Official Airline Guide; the electronic version; the airlines' electronic timetables, and the airline computer reservation systems.

For a trip from Copenhagen to Houston, for example, the information is pretty clear.

The OAG Electronic Edition shows Scandinavian Airlines' Flight 911 connecting with Continental Flight 767 in Newark.

Directly beneath it is a listing for Continental Flight 8911 from Copenhagen connecting in Newark with the same flight to Houston. A footnote says "CO 8911 operated by SAS." In fact, 911 and 8911 are the same thing. In the OAG book, Continental 8911 is marked with a star indicating "dual designated carrier," and one can figure out the name of the carrier because the listing above, for 911, credits SAS.

In the case of the Lufthansa-Air Mauritius flight, the printed OAG lists both legs of the trip as Lufthansa flights; there is no indication that Lufthansa shares its code with another line.

According to Lucille Hobshabjian, a spokeswoman for Lufthansa, the line's electronic timetable for the time the ticket was booked listed the flight twice, with the Frankfurt-Mauritius leg entered with the LH of Lufthansa and with the MK of Air Mauritius.

However, the Lim-Cheng tickets were sold from an electronic directory maintained by Sabre, one of the big four computerized airline reservations systems. There, the two entries for the flight were widely separated by other entries.

The tickets were sold by the Flushing Travel Agency, a branch of Orient Clipper Travel of 217 Park Row in Manhattan, a consolidator, or agent for sale of deeply discounted tickets bought at wholesale from the airlines. Rudolph Den, the owner, said that the first-class Mauritius tickets were not discounted, but that the Long Islanders had been customers for some years and came to him for the tickets anyway.

The tickets were issued by Lanny Wang, an assistant who said that on the basis of the Sabre data, she had assumed that the flight was Lufthansa all the way and did not learn differently until Dr. Cheng returned from the trip and complained. She said that she did not know that Lufthansa owned no 767's.

Inder Sethi, the manager for North America for Air Mauritius, said that the airline, which owns two planes, both 767's with 12 first-class seats in two rows, had many and varied agreements with other airlines serving his country. He ran through a list, and volunteered that the situation was confusing for the traveler.

As for telling the Lufthansa ticket-holders that they are going to arrive aboard Air Mauritius, he said: "I doubt if the agents are aware."

Once again, even with the federal policy, the burden is on the traveler. If it matters, he or she should ask the travel agent to call and find out whose plane will be used.