Saturn is the planet being described
Capricorn belongs to Saturn, which in the old vocabulary means limit, structure, delay and the price attached to things. Nearly every warning written about Capricorn pairings is a restatement of Saturn's reputation: cold, slow, unwilling to be hurried, more interested in what holds than in what pleases.
Saturn is also the single most useful body in a real comparison, and not because of Capricorn. One person's Saturn falling on the other's Moon or Venus is the contact traditional synastry treats as weighty, and it turns up between any two sun signs whatever, including the ones on the recommended lists.
Three hard angles, and no others
Cancer lies 180 degrees from Capricorn; Aries and Libra lie 90 degrees away either side. That exhausts the hard aspects, so the difficult list has three names by arithmetic.
All four are cardinal signs opening their seasons, and each expects to set the terms. The friction is therefore about order of operations rather than about values: everyone is prepared to act and nobody has agreed who goes first.
The Cancer axis is a division, not a defeat
Capricorn and Cancer split one subject between them: what is built in public and what is kept at home. Traditional texts read the axis as a distribution of responsibilities, which is why partnerships across it are commonplace rather than remarkable.
Whether that distribution feels like a partnership or a grievance is decided by the two people. The geometry supplies the topic and stops there.
What squares are, and what lists cannot see
A square is 90 degrees between signs of the same mode and unlike elements. Hard, in this vocabulary, describes an aspect that forces adjustment: something has to give, and generally something does. Aries squares Capricorn with Mars, Libra with Venus, so one square is about pace and the other about consultation.
The severity depends on how close the angle is, and two birth months carry no such information. Alongside orb, the contacts that genuinely indicate strain are Neptune on a personal planet and hard Mars-to-Mars aspects that describe how a disagreement escalates. The synastry calculator measures the actual angles, and synastry sets out what matters.
The pair page does more work
Capricorn and Cancer is written up as a combination, with what the two construct set beside what erodes. That is considerably more use than a rank in a list of worst matches.
If both birth times exist, compare the charts and check whether the alleged problem is present at all.
Questions people ask about this
Which signs are named hard for Capricorn?
Cancer opposite, Aries and Libra square. All three are cardinal, as Capricorn is.
Is Cancer a bad match for Capricorn?
Not by the angle. The axis divides public and private responsibility, which either party can find useful or irritating.
Why is Saturn worth checking in any comparison?
Saturn from one chart on the other's Moon or Venus is read as heavy, and it can occur between any two sun signs.
What predicts a difficult pairing?
Contacts between personal points with tight orbs, not sun signs or element clashes on their own.