Durability and attraction are different questions
Most compatibility tools answer one question: is there chemistry. That is a real question and it is not the one people ask when they are deciding whether to build a life with someone.
This page looks only at the second. Charts that light up on attraction and charts that hold together over decades are not always the same charts, and separating the two makes both easier to see.
What it reads
Three things, chosen because they are what the tradition actually associates with lasting commitment rather than with romance.
Saturn contacts. Saturn between two charts is the most reliable marker of durability in the literature, and the most consistently reported by couples. It feels restrictive early and load-bearing later, which is what Saturn does everywhere else.
Juno links. The asteroid named for the goddess of marriage is read for the terms a person needs a long partnership to meet. Contacts to it describe whether those terms are being offered.
Seventh and eighth house overlays. Where one person's planets fall in the other's seventh house describes what they bring to the partnership itself; the eighth covers what gets entangled, financially and otherwise.
Why Saturn is not bad news here
Saturn's reputation makes people uneasy when it shows up in a comparison. In synastry it is the opposite of a warning.
Relationships with no Saturn contact at all are frequently pleasant and structurally weightless: nothing holds them when circumstances change. Relationships with a hard Saturn contact are harder work early and considerably more likely to survive a decade.
What genuinely predicts trouble is Saturn alone, with nothing supportive alongside it. The full contact list underneath the reading is there so you can check which case you are in.
What it deliberately leaves out
Venus and Mars contacts, which describe attraction and are covered by their own tool. A compatibility score, which the compatibility calculator produces. The composite chart, which describes the relationship as an entity and has its own page.
Leaving those out is the point. A page that shows everything shows nothing in particular, and the contacts here are narrow enough that an empty result is informative rather than a failure of the tool.
When the result is empty
It happens, and the page says so plainly rather than widening the orbs until something appears. Two charts with no Saturn contact, no Juno link and nothing in each other's seventh houses are not doomed; they simply do not carry the specific markers this page looks for.
Plenty of long marriages have thin commitment signatures, and plenty of charts full of them belong to people who never married. The technique describes texture, not outcome, and no configuration obliges anyone to do anything.
What no chart can tell you
Whether to marry someone. Whether a marriage will last. When it will happen, or to whom.
Astrology has a long history of claiming otherwise and a poor record on the claim. What synastry does reasonably well is describe what a relationship is made of, which is useful information and a different thing from a prediction.
Both birth times matter here
More than for most comparisons. The seventh house overlays are half of what this page reads, and houses need an hour. Without both times you get the Saturn and Juno aspects and nothing else.
If only one of you has a recorded time, one direction of overlay is still available, and the calculator will show it rather than dropping the section entirely.
Common questions
What does this read that a compatibility score does not?
Durability rather than attraction. Saturn contacts, Juno links and the seventh and eighth house overlays, which are what the tradition associates with lasting commitment.
Is a Saturn contact bad news?
The opposite, in synastry. Relationships with no Saturn contact are often pleasant and structurally weightless. What predicts trouble is Saturn with nothing supportive alongside it.
Can a chart say whether we should marry?
No. Nothing in astrology predicts whether a marriage happens or lasts. What synastry describes is what a relationship is made of, which is a different kind of information.
Why is my result empty?
Because these two charts do not carry the specific markers this page looks for, and the page says so rather than widening the orbs until something appears. Plenty of long marriages have thin commitment signatures.
Do we both need birth times?
For the house overlays, which are half of what this page reads, yes. Without them you get the Saturn and Juno aspects only. If one of you has a time, one direction of overlay still works.
Where do Venus and Mars go?
To the Venus and Mars comparison, deliberately. Attraction and durability are separate questions and mixing them is what makes most compatibility readings vague.