Norwegian new seats 16.8 inches wide!?!?!

[DaveReidUK]

Originally Posted by underfire

Delta seat widths per ac...https://www.delta.com/content/www/en_US/traveling-with-us/airports-and-aircraft/Aircraft/boeing-737-900er-739.htmlI
You must also check where the measurement is from. Some use center of armrest to center, others use distance between armrests

I think it's pretty universal to quote "seat width" as the measurement between the inside faces of the armrests, as per B ("pax width") in the diagram.

The distance between the centres of the armrests, somewhat disingenuously described in the diagram as "hip clearance" (typically 2 inches more than B for a Y seat) only becomes relevant if you're having an armrest war with the passenger next to you.