Monocoque (French for "single shell") or unibody is a construction technique that uses the external skin of an object to support some or most of the load on the structure. This is as opposed to using an internal framework (or spaceframe) that is then covered with a non-load-bearing skin. Monocoque construction was first widely used in aircraft, starting in the 1930s, and is the predominant automobile construction technology today.
Prior to this time aircraft were built up from an internal frame, typically of wood, which was then covered (or skinned) with fabric to give it a smooth surface. The materials vary; some builders had experimented with the use of metal tubing for the framing, and a number had used thin metal sheets or plywood for the skin. In all of these designs the idea of load-bearing structure vs. skin remained.
By the late 1920s the price of aluminium (specifically duralumin) started dropping considerably and many manufacturers started using it to replace the internal framing, and in some cases, the external skin. A classic example of such a design is the Douglas DC-3, which is an "old style" plane built of new materials. The structure of the plane consists of a latticework of U-shaped aluminium beams, with a thin skin of aluminium riveted on top.
When these designs started appearing it was realized that the skin itself had significant structural properties of its own. With a sufficient thickness, one could do away with all of the internal structure. However this would be even heavier than the framing would have been. At thinner gauges the skin could easily provide the structure for tension and shear loads (metal resists being pulled apart quite well), and if it was bent into a curve or pipe, it became quite strong against bending loads as well. The only loading it could not handle on its own – at least for thin "skins" – was compression. Combining this sort of structural skin with a greatly reduced internal framing to provide strength against compression led to what is known as "semi-monocoque".
The result was a structure that was just as strong as ones made with older methods, but that weighed considerably less. For aircraft construction this is a very important consideration. At the beginning of WWII the technique was just starting to appear, and many aircraft still used mixed construction. By the end, all planes were monocoque.
The first automotive application of the monocoque technique was 1923's Lancia Lambda. Citroën built the first mass-produced monocoque vehicle in 1934, the innovative Traction Avant. The popular Volkswagen Beetle also used a semi-monocoque body (its frame required the body for support) in 1938.
In the post-war period the technique became more widely used. The Ford Consul introduced an evolution called unit body or unibody. In this system, separate body panels are still used but are bolted to a monocoque body-shell. Spot welded unibody construction is now the dominant technique in automobiles, though some vehicles (particularly trucks) still use the older body-on-frame technique.
In automobiles, it is common to see true monocoque frames, where the structural members around the window and door frames are built by folding the skin material several times. In these situations the main concerns are spreading the load evenly, having no holes for corrosion to start, and reducing the overall workload. Compared to older techniques, in which a body is bolted to a frame, monocoque cars are less expensive and stronger.