The zodiacal signs are divided into four groups called "triplicities," and in their order they represent the four elements of fire, earth, air, and water.
Fiery Triplicity is composed of Aries, Leo, Sagittarius
Earthly Triplicity: Taurus, Virgo, Capricorn
Airy Triplicity: Gemini, Libra, Aquarius
Watery Triplicity: Cancer, Scorpio, Pisces