Mars is the sign under discussion
Every claim on this page runs through Mars, because Mars rules Aries and Mars is the planet of appetite, initiative and the willingness to force an issue. When a list says Aries clashes with Cancer, what it is really asserting is that Mars behaves badly in the vicinity of the Moon, and that is a statement about two planets rather than about two birthdays.
Which is convenient, because Mars and the Moon can be measured. In any two charts you can see exactly where they fall and how close the angle between them is, and that measurement holds whatever the sun signs happen to be.
The three signs at hard angles
Libra stands 180 degrees from Aries, directly across the wheel. Cancer and Capricorn stand 90 degrees away on either side. No other sign forms a hard angle to Aries, which is why the list of difficult signs is always these three and never anything else.
The Libra axis is the one about self and other: Aries decides and Libra consults. Cancer and Capricorn are both cardinal like Aries, so all three want to be the one who starts things, and the friction is a scheduling dispute more than a temperamental one.
An opposition is a shared axis, not a wall
Two signs at opposite ends of one axis want the same territory approached from either end, which is why a great many durable partnerships sit across an axis rather than along an easy angle. Aries and Libra is the textbook case: the same subject, initiative and its consequences for other people, split between two people who each hold one half of it.
Whether that division is experienced as relief or as a running complaint is not determined by the geometry. The angle sets the subject; the people decide the tone.
What a square is, and what it is not
A square is 90 degrees between signs sharing a mode but not an element. The old word for it is hard, and hard in this vocabulary means productive of friction, not destructive. Something in the arrangement has to give, and usually something does.
Orb is the part the lists cannot represent. An Aries sun at one degree and a Capricorn sun at twenty-eight are nowhere near a real square, and a chart comparison would treat them as unaspected. Sun-sign lists have no mechanism for noticing this and so report a clash that is not present. Real strain shows up instead in Saturn from one chart landing on the other's Moon, in hard Mars-to-Mars contacts that describe how a row escalates, and in Neptune blurring a personal planet. The synastry calculator measures those angles with their orbs, and synastry sets out what the technique weighs.
Read the pairing itself
Aries and Cancer has its own page, and it describes what the two manage well alongside what they wear through, which no ranking can do. That is worth more than knowing where a sign places on a list somebody assembled from angles.
With both birth times available, compare the charts directly and see whether the alleged clash is even there.
Questions people ask about this
Which signs are the hardest angles for Aries?
Libra opposite, Cancer and Capricorn square. The tradition calls these hard rather than worst.
Is Libra a bad match for Aries?
Not on the geometry alone. Opposite signs share an axis and a mode, and the tradition reads that as attraction at least as often as friction.
Why does orb matter?
A square that is eight degrees wide behaves very differently from an exact one, and sun-sign lists cannot tell the two apart.
What actually predicts a difficult pairing?
Saturn or Neptune from one chart on the other's personal planets, hard Mars contacts, and tight orbs.