Originally Posted by
chornedsnorkack
The seat manufacturer and pitch are up to airline. Width - not so much so. The basic shell gives the total width, and sidewall shape. The airline can then choose the number of seats abreast - which has to be integer. That gives the seat width
Not quite so. For example the A320 is 15cm (6") wider than the 737/757, which most airlines translate into a 2.5cm (1") wider seat. A few however (Northwest is one) choose to fit their A320 with the same seating units as in their Boeings, getting a notably wider aisle but no seated passenger benefit for the extra fuselage width.