The two bright stars of the constellation carry the names of the Greek twins Castor and Pollux, the Dioscuri, sons of Leda, one mortal and one immortal, who chose to share their immortality between them and were made protectors of sailors. The Babylonian lists already saw two figures here, MUL MASH-TAB-BA-GAL-GAL, the Great Twins, identified with the gods Lugalirra and Meslamtaea, gatekeepers of the underworld. The pairing of a mortal and an immortal brother is the element the later tradition made most of, and it is the reason the sign is so often given the sense of a divided or double nature, one part turned toward earth and one toward heaven. Castor and Pollux are also astronomically unlike: Pollux is a single orange giant with a known planet, while Castor is a multiple system of six stars, which is a pleasing accident rather than an ancient observation.
The Babylonian twins were not always benign in later retellings: some Greek sources make the pairing of a mortal and an immortal brother a source of grief as much as devotion, since Pollux's choice to share his immortality is prompted by Castor's death, and the myth is as much about loss as about loyalty. Manilius, the first-century Latin didactic poet whose Astronomica is one of the few surviving verse treatments of the zodiac, dwells on the sign's double-natured restlessness at some length, reading Gemini as producing minds suited to letters, invention and the mastering of several skills at once rather than any single vocation.