A ménage à trois (French for "household of three") is a domestic arrangement in which three people having romantic and/or sexual relations with each other occupy the same household. It is a form of polyamory.
History has a number of examples of ménages à trois relationships. Sir William Hamilton (British ambassador to Naples), his wife Emma Hamilton, and her lover, the naval hero Admiral Horatio Lord Nelson, were in a ménage à trois from 1799 until Nelson's death in 1805. The German intellectual Dorothea von Rodde-Schlözer, her husband Mattheus Rodde and the French philosopher Charles de Villers also had a ménage à trois from 1794 until her husband's death in 1810. At the age of 16, in 1813, the future author of Frankenstein, Mary Godwin, eloped with her to-be husband Percy Bysshe Shelley and engaged in a ménage with Claire Clairmont, future lover of Lord Byron, with whom the Shelleys would later have an extensive relationship. As did the writer Aldous Huxley and his first wife Maria, with Mary Hutchinson, a friend of Clive Bell. The author E. Nesbit lived with her husband Hubert Bland and his mistress Alice Hoatson, and raised their children as her own. The actress Hattie Jacques lived with her husband John Le Mesurier and her lover John Schofield. The political philosopher Friedrich Engels lived in a ménage à trois with his mistress Mary Burns and her sister Lizzie.
Speculation exists that, in 1547–48, Queen Catherine Parr, widow of Henry VIII, and her fourth husband Thomas Seymour were involved in a ménage with the future Queen Elizabeth. This is probably exaggerated, although episodes of sexually charged horseplay involving the three were well attested. The Russian and Soviet poet Vladimir Mayakovsky lived with Lilya Brik, who was considered his muse, and her husband Osip Brik, an avant garde writer and critic.
In 1913, the well-known psychoanalyst Carl Jung began a relationship with a young patient, Toni Wolff, which lasted for some decades. Deirdre Bair, in her biography of Carl Jung, describes his wife Emma Jung as bearing up nobly as her husband insisted that Toni Wolff become part of their household, saying that Wolff was "his other wife". Erwin Schrödinger, his wife Annemarie Bertel, and his mistress Hilde March had a ménage à trois.