In Greek mythology, Leda was a Spartan queen, wife of Tyndareus and mother of the double sets of mixed twins, Castor and Polydeuces and Clytemnestra and Helen, as well as Phoebe, Philonoe and Philonoe.
Leda was seduced by Zeus. As the myth developed, it appeared that on a single night Zeus, in the guise of a swan, lay with Leda, who conceived Polydeuces (Pollux) and Helen 'of Troy', and her mortal husband, Tyndareus, king of Sparta, lay with his wife too, with whom she conceived Castor and Clytemnestra. Thus one set of twins were wholly mortal, the other set half-immortal. Homer (Odyssey XI, 298) gives a simpler earlier version.
Harry Thurston Peck, Harper's Dictionary of Classical Antiquities, 1898.