Venus rules Libra, and rules Taurus too
The single most useful fact about Libra is one the compatibility lists never mention: Venus governs two signs, and it behaves quite differently in each. In Taurus it is about keeping and enjoying; in Libra it is about balance, proportion and the presence of another person to measure against. Libra's famous difficulty with deciding is not indecision so much as an unwillingness to settle a question that a second party has not weighed in on.
So the question that matters in any Libra comparison is where the other person's Venus falls and whether it wants the same currency. A Venus in Scorpio does not offer affection in the register a Venus in Libra recognises, and neither party is doing anything wrong.
The four traditional names
Aquarius and Gemini complete the air trio, each a third of the circle from Libra. Sagittarius and Leo are fire, a sixth of the circle away on either side. The two spacings are the ones old practice ranked as easy, and the sign names are simply whatever stands at those distances.
Stated plainly, the list is a fact about a wheel with a fact about people attached to it afterwards. There is no survey of Libra partnerships underneath it and there never was.
Cardinal air, which the caricature misses
Libra opens a season, which makes it an initiating sign, and popular writing consistently reads it as passive instead. What it initiates is contact: the introduction, the invitation, the terms of an arrangement. Set against fixed Aquarius, the tradition expects principle meeting diplomacy. Against mutable Gemini it expects two people who will discuss a decision more happily than they make one.
Both are reasonable sketches of how the categories combine. Neither predicts anything about two named individuals, and neither pretends to have been tested.
The comparison that has content
Reading two suns against each other uses a fraction of the available information. A real comparison looks at both Moons for what each person needs when unobserved, both Venus placements for how fondness is given and read, both Mars placements for how a row develops, and both ascendants for the manner each leads with, then measures the angles running from one chart to the other. That measurement is synastry; the synastry calculator carries it out from two birth times.
What it cannot supply is a soulmate. Astrology has no term for the idea, no placement that signals it and no classical basis for the claim.
Reading a pair properly
Take the sign you are actually curious about and read that combination whether or not it appears among the recommended four. Libra and Cancer, a square, is a more searching read than Libra and Gemini, since the page has to work out what the two modalities do to one another.
Then compare the charts if both birth times exist. It is a narrower exercise than the one people expect and the only one that stands up.
Questions people ask about this
Which signs are named for Libra?
Aquarius and Gemini in air, Sagittarius and Leo in fire. All four are consequences of the two angles that classical practice reads as easy.
Can astrology find a soulmate?
No. The concept has no technical meaning in the subject and no placement corresponds to it.
Why is Venus central to a Libra reading?
Venus rules Libra, and it works differently there than in Taurus, its other sign: proportion and partnership rather than keeping and enjoying.
What replaces sun-sign matching?
Moon, Venus, Mars and ascendant in both charts, and the aspects that run between the two charts.