Gemini is the third sign and the first of the mutable or common signs, the ones that fall at the end of a season and carry it over into the next. It is airy and ruled by Mercury, and has no traditional exaltation of its own in the standard scheme. The mutable quality is read for adaptability, plurality and a tendency to divide attention; the Mercurial rulership for speech, exchange, short journeys, learning and the handling of information. Ptolemy notes the changeable and duplicated character of a sign figured as two persons, and the classical texts class it as a bicorporeal or double-bodied sign, a category that also governs questions about twins, partners and matters that come in pairs or recur. As with all the signs, the earliest sources tie these meanings to season and figure rather than to any theory of temperament.
Gemini's three decans, in the Chaldean order used since Hellenistic times, run Jupiter, Mars, Sun for the first, middle and final ten degrees, a sequence obtained by continuing the fixed planetary cycle Saturn, Jupiter, Mars, Sun, Venus, Mercury, Moon unbroken across all thirty-six decans of the zodiac from zero degrees Aries rather than restarting within each sign. As an air sign Gemini also carries a triplicity rulership, Saturn by day and Mercury by night in the table used by Dorotheus and later by William Lilly, one of the three dignities, alongside domicile and exaltation, that classical horary and electional astrology weighed in judging a planet's strength.