The Marquise de Sevigne wears a hurluberlu coiffure, probably supported by wire or equivalent arrangement, in this portrait by Robert Nanteuil.
The Wikipedia article for Robert Nanteuil is here.
This is a classic hurluberlu coiffure. As a widow, she wears an attifet.
Keywords: Nanteuil, Marie de Rabutin-Chantal, Marquise de Sévigné, de Rabutin-Chantal family, Sévigné family, Marquise, French, hurluberlu coiffure, attifet, off shoulder scoop neckline, bertha, bows, necklace