in the late 1960's, there was a simplicity patttern that had gussets at the waist, of a semi fitted dress..

the suggestion was for a light main color, and a dark constrasting color..

the waist gussets were diamond shaped, and pointed in.. something like this.
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sort of shape at the waist, with the side panels in a dark color..

it whittled inches off the waist visually!

Most commonly, i have seen (and used gussets) in the underarm area, to give a neat underarm in a dolman or cap sleeve.

I never thought about them in theatrical scenery!