Saturn is the ruler, and Saturn is slow
Capricorn belongs to Saturn, which takes about twenty-nine and a half years to go round the zodiac and stands, in classical practice, for limit, structure and the cost of things. Saturn is the reason the Capricorn caricature reaches for words like reserved and durable, and it is also the reason those words describe so little: a planet that spends two and a half years in a sign is shared by everyone born in that stretch.
Where Saturn earns its keep is in comparison rather than description. One person's Saturn falling on the other's Moon or Venus is the contact that traditional synastry treats as weighty, and it can appear between any two sun signs at all, listed or not.
The traditional four, and where they came from
Taurus and Virgo are the other earth signs, each a third of the circle from Capricorn. Pisces and Scorpio are water, a sixth of the circle away either side, and water is held to soften earth. Those separations were classified as benign in antiquity and the sign names simply occupy them.
What that produces is a list identical for every Capricorn who has ever lived, which is a fair indication of how much it can be describing any of them.
Cardinal earth, which is not the same as immovable
Capricorn opens a season, so it initiates, and the element it initiates in is the one concerned with durable results. The combination is ambition with a timetable rather than ambition with a temper. Against fixed Taurus the tradition expects a shared preference for arrangements that hold. Against mutable Virgo it expects standards agreed and methods argued about.
These read as descriptions of a working arrangement, which is what they are. Whether two particular people find the arrangement companionable is outside the reach of the categories.
What a two-chart reading actually weighs
One sun compared against another uses a sliver of what is available. The comparison with substance reads both Moons for private requirement, both Venus placements for how warmth is offered and understood, both Mars placements for how disagreement proceeds, and both ascendants for the manner each person leads with. Then it measures the aspects between the two charts rather than within either, which is synastry, and the synastry calculator runs it from two birth times.
What no version of it does is locate a soulmate. The term is not astrological, no placement signifies it, and using it borrows authority the subject has not got.
Reading a pairing rather than a place on a list
Go to the combination you are genuinely wondering about. Capricorn and Aries, a cardinal square, describes the difficulty in detail and is more useful than a page confirming that Capricorn and Taurus get on.
Then, with both birth times available, compare the charts. The answer is smaller than the question people arrive with and considerably more honest.
Questions people ask about this
Which signs are named as easy for Capricorn?
Taurus and Virgo in earth, Pisces and Scorpio in water. The grouping is a consequence of the angles rather than of observation.
Does astrology identify soulmates?
No. There is no placement or aspect that carries the meaning, and the tradition makes no such claim.
Why does Saturn matter in a comparison?
One person's Saturn contacting the other's Moon or Venus is read as weighty, and that contact is independent of the two sun signs.
What should be read instead of sun signs?
Both Moons, Venus, Mars and ascendants, and the aspects that run between the two charts.