When in economy on long flights on 757s, I like the section right behind the restrooms (mid-cabin) for easy access to them, as well as a bit more space as usually everyone wants up front seats. This makes less difference on short trips, as you naturally don't have as much time to 'stretch out.' Also, I always consult seating maps on airline www sites and in the timetables. I rarely pick a seat without consulting these, unless it is a last minute at-airport change.

Also, exit-row seats almost always have more legroom.

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See you at the airport!