In "Astrology's four elements: Fire, earth, air & water" you'll learn that the elements divide the zodiac signs into four groups, each of which contain three signs. The elements describe four very basic and distinct personality temperaments. But each of the twelve astrological signs also has a mode of operation, or modality.
While the elements divide the twelve signs of the zodiac into four groups of three signs and the modalities divide the twelve signs into three groups of four signs. Each of the astrological signs has a distinct temperament and distinct way of expressing that temperament.
For example, consider the fire signs, Aries, Leo and Sagittarius. All three have fire in common, but each is unique in the way it expresses it's fire. Aries provides the spark that lights the fire. Leo maintains the fire and radiates its warmth, while Sagittarius adapts the fire to changing conditions and spreads it around. These different behaviors arise because each sign manifests it's fire through a distinct mode, modality or quality of expression.
In short: The elements describe the energy available to each sign, and the modalities depict how the energy moves or acts or behaves.