Mercury rules Gemini and Virgo, is exalted in Virgo, and is in detriment in Sagittarius and Pisces and in fall in Pisces. It is the only classical planet the tradition calls common or convertible: Ptolemy describes it as taking on the nature of whatever it is joined to, dry with the dry and moist with the moist, diurnal when it rises before the Sun as a morning star and nocturnal when it sets after it as an evening star. Because it never appears more than about twenty-eight degrees from the Sun, it is always in the same sign as the Sun or in one of the neighbouring signs, which sharply constrains the aspects it can make. Its natural significations are speech, writing, calculation, trade, messengers, short journeys, the hands and the nervous system, and it is the planet of what passes between things rather than of any thing in itself.
The Latin name Mercury renders the Greek Hermes, who in turn renders the Babylonian planet-god Nabu, patron of writing and the scribal arts, a chain of translation that carried the planet's association with language and communication across three languages and well over a thousand years before it ever reached a Roman text. Ptolemy's Tetrabiblos, compiled in the Hellenistic synthesis of roughly the first century BCE to the second century CE, explains Mercury's changeable elemental quality on physical grounds, drying and moisture-absorbing by turns, and humidifying, because it stands next above the sphere of the Moon, the same reasoning by proximity the text applies throughout. The underlying tracking of Mercury as a celestial omen, distinct from the horoscopic use later built on it, reaches back to Babylonian celestial omen-reading of perhaps 1000 BCE.