One explains and the other feels, and each finds the other strange. Under the elements rule, Gemini is air and Cancer is water, and the rule rejects this pair. Keep that in mind and read on, because the rule answers far less than it seems to.

Gemini brings the world and Cancer brings the home, and when this pair works, it works like this: Cancer builds the nest and Gemini fills it with people, subjects and news. Cancer tends to find Gemini's lightness funny, it lifts the weight of a bad day better than any advice, and Gemini finds in Cancer the one person who cares about what he told her yesterday.

The table every magazine publishes is simple: fire matches fire and air, earth matches earth and water. It was not invented yesterday, and it has a geometric reason, signs of the same element sit 120 degrees apart, and allied elements sit 60 degrees apart. Those are the trine and the sextile, the two aspects that tradition calls harmonious. The compatibility table is the aspect table applied to a single planet.

It puts half the zodiac on each person's list, which already shows how little information that is. And it can be tested against the numbers. I drew a random sample of 600,000 pairs of birth dates between 1966 and 2025, using the same ephemeris that builds charts on this site, and asked: in the pairs the rule approves, do the other pieces follow along?

In the pairs the rule approves, the Moons match in 49.8% of cases; in the ones it rejects, in 50.0%. There is contact between one person's Sun and the other's Moon in 46.4% of both groups, and contact between Venus and Mars in 46.1% and 46.2%. Between the two charts there are, on average, 26.7 contacts in the approved pairs and 26.6 in the rejected ones.

The columns are the same. Knowing that the rule approves or fails your pair tells you nothing about the other person's Moon, nothing about desire, and nothing about how many contacts the two charts have. And 50.0% is exactly what a coin toss would give you.

That does not make the table false. It makes it small: it describes a trait of temperament, read from a single point on the chart. Using it to decide who to date is reading a book by its title's first letter.

Neighbouring signs do not see each other. They are 30 degrees apart, the semi-sextile: too close for contrast and too far for recognition. Cancer processes what she feels and takes her time; Gemini processes by talking and has already moved to another subject. So Cancer feels dismissed in the middle of something important, and Gemini did not understand it was important, for him, talking about it already counted as caring about it. Neither one is behaving badly, and both end up hurt.

What gives this pair its ground is the Moon, and it rules Cancer by domicile. Gemini's Moon decides whether he can stay put in a conversation that is not moving forward, a Moon in a water sign in his chart changes the whole pair, because it gives Gemini patience for the other's pace.

What usually unblocks things: Gemini staying put. Not fixing, not explaining, not offering three ways out, just staying there while Cancer finishes feeling what she feels. It is the cheapest gesture this pair has, and the one that least occurs to Gemini, who helps by thinking things through. And Cancer gains by putting the complaint into words, even badly formed ones: Gemini reads text far better than he reads mood, and what was not said never arrived.

The element rule reads one thing only: the element. Each sign carries two other labels the table ignores, and in both cases they say more about the day-to-day than the element does. The modality, cardinal, fixed or mutable, is the way a person goes into a subject: cardinal starts it, fixed holds it, mutable changes it. And the ruler is the planet that gives the sign its vocabulary.

Gemini is mutable and ruled by Mercury; Cancer is cardinal and ruled by the Moon.

Mutable with cardinal, and the cardinal one is the water sign: Cancer does not start by arguing, she starts by taking care, she proposes the home, the habit, the change, from the inside. Gemini goes along and does not notice that was a proposal, because for him a proposal is the sentence that says «let's do this». A lot of this pair's decisions happen without Gemini knowing a decision was made.

Mercury and the Moon are the most unequal pair when it comes to memory. Mercury keeps the fact and forgets the tone; the Moon keeps the tone and forgets the fact. A fight in this pair has two versions that never meet: Gemini remembers exactly what was said and Cancer remembers exactly how it was said, and each thinks the other is making it up.

And there is a difference in recovery speed. Gemini forgets an argument within two hours and thinks it is over; Cancer takes days, and Gemini's cheerful return the next day is not read as reconciliation, it is read as proof that it did not matter.

There is one exception to "the table tells you nothing", and it comes down to pure geometry. Venus never moves more than 47.2 degrees from the Sun, it is an inner planet, and from Earth's point of view it stays tied to the Sun's light. Given your sun sign, your Venus can only fall in five signs.

So the distance between two sun signs decides the odds that both Venuses land in the same sign. Measured across the same 600,000 pairs:

When both are of the same sign, which is the conjunction, the two Venuses fall in the same sign in 24.3% of pairs; one sign apart, the semi-sextile, in 19.2%; two apart, the sextile, in 12.9%; three apart, the square, in 5.5%; four apart, the trine, in 0.8%; and five or six apart, the quincunx and the opposition, never.

Gemini and Cancer are 30° apart, a semi-sextile, and that puts this pair in the 19.2% bracket. The two of you may well have Venus in the same sign, and it is the one affinity your sun sign guarantees, at least a little.

Notice what the table does with the element rule: it cannot follows the rule. Pairs the rule fails come out on top of the list, and pairs it approves come out at the bottom. The Venus link is real, and it is geometric, and it is why the compatibility table sometimes seems to work. It works a little, and not for the reason it claims to.

Four pieces that change everything, and that no sun-sign table can reach:

• Each person's Moon. It is the way you feel, need and settle down, the piece that most decides day-to-day life together. It changes sign every two and a half days.
• The Ascendant. It changes every two hours and depends on the city. Two Geminis born on the same day can arrive at a party in opposite ways.
• The house each one falls into on the other's chart. The same Venus in your house 5 and your house 12 is passion in one case and a private matter in the other. This layer needs both people's birth time.
• What each one has already lived. Age, past relationships and the timing of the meeting weigh more than any aspect, and none of it is in anyone's chart.

This crossing is what's called synastry, and it needs both people's date, time and city of birth. Comparing sun signs is what's left when you don't have that.

And the limit stated out loud still holds: astrology decides nothing for anyone, doesn't tell you to stay or leave, and doesn't replace conversation or therapy. It gives you vocabulary. A couple who discover that one Moon needs routine and the other needs novelty have gained a better question than they had before.

And the questions that come up most on this subject, with the short answer: