If your airplane ticket says Row 13 on a US Airways flight between Philadelphia and Cleveland, I recommend that you investigate who’s flying the plane. By who, I mean which airline.

In this world where airlines share codes, you may not know who your actual carrier is, unless you pay attention. Delta can be Continental, and Continental can be Northwest. Most of the time it doesn’t make a lot of difference: You buy one ticket, fly from place to place and get about the same level of service.

The airlines say code sharing is a convenience for passengers. I say it is a convenience for the airlines, and passengers should be wary.

When codes are shared between airlines with big jets and regional carriers with small ones, differences can be substantial. You need to find out about the airplane — and sometimes about the airline.

For instance, if you buy a ticket on US Airways between Philadelphia and Cleveland, you may think that a seat in Row 13 is near the middle of the plane.

But not on a Canadair CL-65 jet in the Air Wisconsin fleet. Air Wisconsin operates a US Airways Express flight between Philadelphia and Cleveland.

If you are in Row 13 on this 50-seat jet, you will be sitting in the last row on the left side of the plane. How do I know it’s the left side? Because on the CL-65, the lavatory fills Row 13 on the right side.

On my recent flight from Philadelphia to Cleveland on Air Wisconsin, the odoriferous lavatory in Row 13 (right) spread its cheer across the tiny aisle.

After more than 30 minutes on the runway, we flew about an hour to Cleveland, which gave me ample opportunity to analyze the quality of the air. Row 13 did not improve as we headed west.

Be careful out there: Stay away from airlines that fail to keep their lavatories in top smelling condition. Factories have been closed with cleaner air than I breathed on US Airways flight 3610.

And I hope that you do a better job than I did of paying attention to the size of the plane when you get your seat assignments. One useful site is www.seatguru.com, which shows every airline and every seat on every kind of plane, including the positives and negatives of each seat.

Unless your flight from Philadelphia to Cleveland is some sort of emergency, stay away from smelly Air Wisconsin’s Row 13. I am not far enough away from it yet.

The writer: David Molyneaux is editor of www.TravelMavens.net.