[thackney]

It really depends both on the airline and which model of aircraft.

http://www.seatguru.com/charts/shorthaul_economy.php

[sickoflibs]

For most of us we are mainly interested in domestic carriers.

If they are selling room in the middle of the plane for a Economy+ type type deal then you can bet they are getting that room by screwing the other seat holders,

If the flight is short then its not that big a deal, but two or three hours in a cattle car is painful.

If I have to fly United then I just pay the extra fee, but I try to avoid that carrier altogether,

Seat maps United_Airlines_Boeing_757-200 V Domestic