build length.
When aircraft were "bespoke" there was a large variation in the length/span.
Not usually a problem. That is until some bright spark decides to modify all the Nimrods (basically a Comet 4) up to a new standard; first aircraft in the hangar, all the jigs made and set up, second aircraft in NOTHING fits anywhere. Probably goes a long way to explain the huge cost over run and very very long delay. Or is that me being cynical again