It's the pervasive type of car construction today.
The definition of monocoque construction relates to "single-shell" car construction in which the entire external skin of the monocoque car body is a structural, stress-bearing shell, similar to an eggshell. It is often incorrectly applied to road cars which should rather be referred to as having semi-monocoque or unibody-frame designs.
While unibody and monocoque construction are sometimes used interchangeably, they aren't actually the same. We define unibody as a unitized vehicle body with tubes, bulkheads, and box sections that provide most of its strength, whereas a true monocoque structure gets its strength from the entire external 'skin'. This stands in contrast to a body-on-frame design in which the ladder frame underneath provides most of the strength and a non-structural body is just bolted on top of it.
The first unibody car was the 1922 Lancia Lambda, but only a thousand were produced. In 1930, Joseph Ledwinka, an engineer at the Budd Company in the USA (ThyssenKrupp Budd today), designed a full unitary prototype body, which French automaker Citroën bought to use for their mass-production 1934 Citroën Traction Avant. Budd supplied a version of this to Chrysler for its 1934 Chrysler Airflow, making these two the first true unitary-design mass-production cars. A true monocoque frame was first used in he 1962 Lotus 25 Formula 1 car, followed by the carbon-fiber-reinforced structure of the McLaren MP4/1 in 1981. In 1990, the Jaguar XJR-15 was developed, becoming the first producton car with a carbon-fiber monocoque.
For normal sedan, hatchback, crossover, and station wagon cars, the unibody design is the most popular way to build a vehicle, although some SUVs and most trucks still use body-on-frame construction today. The monocoque chassis design is mainly used in sports cars and race cars, like the McLaren 720s and Bugatti Chrion.
True monocoque chassis designs are rare in road cars today, but the following benefits are worth mentioning:
In terms of unibody vs. body-on-frame, the differences between the two are what gives the unibody design the following advantages:
That being said, body-on-frame cars have some advantages too:
The body-on-frame SUV concept of the Lincoln Navigator and Cadillac Escalade still holds sway when it comes to the large, hard-working SUV class. And due to being a work vehicle, the full-size F-150 Ford truck and its like-minded rivals will continue to employ the body-on-frame design for a long time still, although a new generation of unibody truck is here for the lifestyle-orientated buyer, notably the Ford Maverick and Hyundai Santa Cruz, which share their unibody construction with normal crossover SUVs. The modern unibody design has taken over as the default format for most cars and SUVs and is safer and more versatile than ever.
A body-on-frame car is made up of more modular elements and it's often cheap to remove and replace body panels, which are often not structural. Also, minor accidents might not damage the frame at all, reducing the cost of crash repairs. A unibody's design is less modular and more integral, meaning that it is not always easy to repair or replace a small section, making accident repairs more expensive in general.
Although many off-roaders stick to body-on-frame designs, a well-designed unibody SUV can be just as capable as the best. This is amply demonstrated by the new Land Rover Defender, which is Land Rover's most off-road-capable product, yet is of aluminum unibody construction.
Most passenger cars today are, but there are exceptions. Currently, the only car (ie: not an SUV or truck) in the US with body-on-frame construction is the BMW i3. Because it is an electric vehicle with a lightweight CFRP body, BMW opted to bolt this body to an aluminum frame that contains the powertrain and battery module, effectively making this a body-on-frame passenger car. The well-known and older Ford Crown Victoria sedan was a popular choice as a police cruiser thanks to its body-on-frame design making for cheap minor crash repairs - an everyday occurrence for police vehicles.
Space-frame cars use a skeletal frame of tubes to bear the entire structure's stresses, with the drivetrain components and suspension bolted to this frame. The body panels that are attached to it often bear no or very little structural stresses. The all-aluminum frame of the Audi A8 is an example of this. This is not regarded as a unibody design, because the body panels are not critical to the structure's integrity.