Determining compatibility between two signs in Western Astrology involves the concept that 12 Western Zodiac Signs are related to 4 different Western Element Groups: Fire (Aries, Leo, Sagittarius), Earth (Taurus, Virgo, Capricorn), Air (Gemini, Libra, Aquarius), and Water (Cancer, Scorpio, Pisces).
Each of the 12 Zodiac Signs is in an element. There are four elements: Fire, Earth, Air, Water. In Sun Sign comparisons, the general rule of thumb (with possible exceptions) is that you tend to get along best with those people whose Sun Signs are in the same element or in a "complementary" element. Fire and Air are considered to be complementary elements, and Earth and Water are considered to be complementary elements.
Signs in the same element are considered to be "trine." If you play "connect the dots" on an astrology chart, "trine" Signs form an equilateral triangle. The exception to this is that people with their Suns in the same Sign (same element) are considered to be in Sign Conjunction.
Signs in a complementary element are considered to be "sextile." If you play "connect the dots" on an astrology chart, "sextile" Signs (together with the "trine" Signs) form a six sided star. The exception to this is that people with their Suns in the opposite Sign (complementary elements) are considered to be in Sign Opposition.
To better understand how this works, let's look at the complementary elements of Fire and Air. Following this rule of thumb — the Fire Signs (Aries, Leo, and Sagittarius) and the Air Signs (Gemini, Libra, and Aquarius) all tend to get along the easiest. But there are two exceptions: conjunctions and oppositions — and astrologers have widely varying opinions regarding the effects of Sun Sign conjunctions and Sun Sign oppositions.
Experience shows that two people with their Sun Signs in the same (conjunction) or in opposite Signs (opposition) get along with one another in either an extremely positive manner or in an extremely negative manner. The key focus here is on extremes. There's generally no emotional middle ground in either a same Sun Sign relationship or in an oppositional Sun Sign relationship.
Another way of classifying and examining the qualities of the various signs is to look at them from the aspect of the star signs and their so-called quadruplicities. The three quadruplicities can be seen as different ways of being creative. The four cardinal, Aries, Cancer, Libra and Capricorn, have in common the impetus to initiate or create, or to counter one initiative with another. The four fixed, Taurus, Leo, Scorpio and Aquarius, are inclined to sustain or stabilize what has already been created, or to resist or practise endurance in the face of another's initiative. The four mutable, Gemini, Virgo, Sagittarius and Pisces, take what has been created and attempt to alter or adapt it.
Combine the four elements with the three quadruplicities, and we find that each of the star signs has a unique combination of the two. For instance, Taurus alone is fixed earth, Sagittarius alone is mutable fire. The four following brief statements regarding the attributes of each star sign are a very brief sketch of the characteristics of each element, and of the differences between each sign of that element on account of the varying quadruplicities.
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