Upgrade image posted 23 March 2015 from Wikimedia; contrast increased. Photo - Micze.
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Christie's auction notes state, "Portrait of Luise Friederike, Duchess of Mecklenburg-Schwerin, née Duchess of Württemberg-Stuttgart (1722-1791), small bust length, wearing a light blue dress with lace chemise and veil, earrings, wig and the sash and the badge of the Russian order of Saint Catherine." I am going to call what Christie's calls a "chemise" a "fichu." Her coiffure is spread out in late 70s or 80s style.
Keywords: Lisiewski, Micze, Luise Friederike von Württemberg, Herzogin zu Mecklenburg-Schwerin, Duchess of Mecklenburg-Schwerin, Württemberg family, Mecklenburg-Schwerin family. German high coiffure, veil, vee neckline, fichu, ruffles, bows, tight sleeves, order sash, order star, earrings