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After 1767, Fragonard's chief work was decorative panels commissioned by Madame du Barry, mistress of Louis XV, for her chateau at Louveciennes. Surprisingly enough, she rejected the panels as unsuitable. This painting was executed shortly before the series, and may have been shown to Madame du Barry as part of Fragonard's "pitch" to win the commission.
In any case, The Love Letter is characteristically muted, with an eroticism that is certainly present, but deeply hidden. What attracts the eye here are the glorious golden colors, and the coquettish attitude of the young lady, rather than body parts. The painting seems to glow with passion.
Can you speculate on the role of the little dog? What is he doing there?