Everyone "knows" what was fashionable in the 1920s - and actually, most people are about
half right.
The low-waisted, pared-down look popularly associated with the twenties was fashionable in the second half of the decade. As you can see from this illustration the look at the beginning of the 1920s was slightly fussier and more feminine. These dresses for dancing are decorated with extra material gathered into side panels and flounces. The look was still very daring for its time, though - a mere decade before women had been swathed in floor-length skirts over multiple petticoats and tightly laced corsets!
Incidentally, F. Scott Fitzgerald was first published in 1920. His early flapper heroines would have worn frocks very like this as they jazzed around the dance floor.