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Gadling's guide to getting better seats: web tools and seat selection
Now you need to check what airplane (or equipment) you're flying on. This should be available from your online itinerary or again via the phone agent. If you go to your carrier's website you should be able to plug in your reference code and last name into the front page and bring up your reservation in addition to a variety of other juicy tibits. You should also be able to change your seat from the online itinerary, but you need to hold off on that for now -- first you need to cross reference your equipment with Seatguru.
Seatguru is an online database of airlines and seat maps across the board. They cover all of the domestic carriers and a few second tier airlines as well, including information on power, Audio and Video On Demand (AVOD) and meal data. It's easier to open up Seatguru in another tab or window so you can quickly cross reference seats later.
You'll see that once you select your carrier a drop-down box of equipment will appear, including the multitude of configurations available for your jet. Since you just checked this online it should be easy to locate your aircraft.
Take a look to see what the best seats on the plane are. These are always highlighted in green and if you hover over them an info box explaining the perks will pop up. Often you'll also find that that bulkhead or exit seat that you wanted is not as good as it looks – many of those rows can't recline or have limited legroom. If you can deal with those restrictions for the other perks however, go for it.
Going back to your itinerary that you brought up on your carrier's website (or calling back the agent at the airline), try selecting new seats from your reservation. Most legacy carriers should allow you do this online, although not all of them permit it. Southwest Airlines, who don't even assign seats to begin with, comes to mind in this case.
If you're able to select the seat right now, great. Grab it and run. But chances are, the ideal seat in the bulkhead, exit row or window are taken right now or "not available". As I mentioned earlier, sometimes airlines will often restrict seats for special flyers or until the last minute. That's next. Right now, select your "worst-case-scenario" seat -- the seat that you'll accept if you can't get that ideal that's taken right now. I like to pick a window seat near the front of the plane.
Continue on to restrictions and openings.