A notchback, unlike a hatchback or fastback, is characterized by a near-vertical drop-off from a car's roof to its trunk. All notchbacks are "three-box" designs – three clearly separate areas for engine, passengers, and cargo. Seen from the side, the section forward of the windshield can be viewed as one box; the section with doors and windows is the second; and the third box is the trunk. Because the third box extends from below the back window, the design is called a notchback. Although notchback is usually a synonym for sedan, many coupés have notchback-type designs as well.
Passenger sedan aerodynamics can affect many areas of a vehicle's performance, such as fuel efficiency, stability, handling, and noise levels. Notchback vehicles exhibit a complicated near-wake flow, the structure of which is still not understood.
As aerodynamic efficiency becomes an ever-greater focus in automobile design, the distinct angle between rear window and decklid that characterizes the traditional "notchback" is gradually diminishing: most of today's four-door sedans feature a long, sweeping roof line that transitions through a shallow curve into a short, more horizontal decklid—i.e. the notchback is vestigial. However, drag-reducing (e.g. streamlined) production automobile design dates from the late 1930s.
The notchback design was common across U.S. automakers and automobile types. A styling trend emerged during the 1960s where rooflines on many two-door models were made smoother with steeper slope of the rear window or more arc (a style that American Motors described as a "modified fastback") whereas, many four-door sedans featured a more upright, elegant roofline. General Motors' intermediate-size two-door models featured a roof lines with a traditional notchback, recessed ("tunneled") rear window between the sailing roof panels. The marketing term "formal roof" was coined for the steeply-angled version seen on certain American cars of the 1980s such as the two-door Mercury Cougar and the C- and G-body cars from General Motors, and the 1985-1991 N-body cars. The "formal roof" styling of the 1970 Oldsmobile Cutlass Supreme hardtop coupe "promised affordable elegance".
The term first became common in British English when used for the European Mark III Ford Escort and the slightly later Ford Sierra, both of which have hatchbacks, but also a residual trunk hump. Officially (in Ford terminology) the shape of these hatchbacks was Aeroback.
In British English a "three-box" sedan is more generally known as a saloon. However "notchback" is also used.