No middle seats!

The MD 88 has 1 middle seats per row, and the Boeings have two middle seats per row.

The Boeings are based on a 1950's design.

The MD -88 are based on a 1960's design.

175s are the peak of seat width comfort. The decline has started with the Boeing 787, which will have 9 across in coach in a plane that is 16 inches more narrow than the 777 that has 9 across.