Most passengers never notice the width difference between B737 and A320 seats. When a major airline did some proper research on the factors that led to perceptions of comfort, seat width was found to have only a marginal effect. Legroom was worth much more (which is how an airline like Emirates can just about get away with 10 abreast in a B777, at a deep pitch).

The most "comfortable" feature of an aircraft, though, was an empty adjacent seat, for which 3-3 seating is good (load factor must be greater than 66% before anyone has to sit next to anyone else). B737 and A320 are equal on this.

But a DC9 layout would mean that load factor must be greater than 80% before anyone has to sit between two other people, although you have to put people next to each other from 60% up.

[Edited to split out and expand the last point for clarity - see below. Apologies.]

[This message has been edited by Globaliser (edited 05-01-2003).]