My research on the Great Depression convinced me that it was two
depressions, occurring one right after the other.  A demand-side
recession that began around September 1929, and a supply-side
depression that began in July 21, 1933 (with another demand shock in
late 1937.)  Because they occurred back-to-back, most saw them as one
“Great” Depression, and looked for explanations of what caused “the”
Great Depression.  No mono-causal theory has proved plausible.