rag tops to hard tops

In 1921 Hudson first mass-produced a closed car.

Almost immediately after World War I, public demand increased dramatically for a closed car that would no longer be a seasonal pleasure vehicle, but rather all-weather transportation.

The few closed body cars built before WWI were extremely expensive and the work of custom coach builders.

 This rise in demand during the 1920s, coupled with a remarkable number of concurrent technical innovations in plate glass and steel manufacture, resulted in a revolution in production methods, productivity and economies of scale.

The transition away from rag tops (the word convertible was first used in 1927 and officially added to the Society of Automotive Engineers lexicon in 1928) was rapid and contributed to a venerable prodigy of production by the end of the 1920s,

 Transition from Open to Closed Cars
 Year    Open Cars (%)    Closed Cars (%)
 1919         90                         10
 1920         84                         16
 1921         78                         22
 1922         70                         30
 1923         66                         34
 1924         57                         43
 1925         44                         56
 1926         36                         74
 1927         15                         85

 Source: John Gunnell, Convertibles: The Complete Story (Blue Ridge Summit, PA: 1984), 129.
Found on http://automobileandamericanlife.blogspot.com/2013/10/the-transition-to-all-steel-automobile.html