Sun-sign compatibility is the postcard version of astrology: charming, portable, and missing almost everything. Synastry is the full letter. It is the traditional practice of laying two complete birth charts on top of each other and studying how one person's planets fall against the other's — which of your placements touch theirs, at what angles, and in which parts of their chart.
The word comes from the Greek for "stars together," and the method is genuinely old: relationship comparison appears in Hellenistic and medieval sources long before magazine horoscopes existed. What the tradition promises is not a score out of ten but a map of texture — where two people find each other easy, where they rub, and where they may not notice each other at all.
This guide is a beginner's route through that map. You do not need to memorize every aspect or asteroid. You need two accurate charts, five checkpoints, and a willingness to read tension as information rather than prophecy.
Start with two real charts
Synastry begins where all serious astrology begins: with birth data. Each person needs a date, a place, and — ideally — a time of birth, because the ascendant and the houses shift roughly every two hours. If you have never cast a chart, our natal chart tool will do the calculation and show you the parts you are about to compare.
With both charts in hand, the classical method is simple to describe: place one chart around the other and look for cross-aspects — significant angles between person A's planets and person B's. A conjunction (0°) blends, a trine (120°) flows, a sextile (60°) cooperates, a square (90°) frictions, an opposition (180°) polarizes. If those terms are new, our guide to zodiac aspects covers them in one sitting.
The five checkpoints to compare first
A full synastry reading can involve dozens of contacts, which is exactly why beginners drown. The tradition's practical advice is to triage: five placements carry most of the relational signal, and comparing them across both charts gives you a workable first portrait.
| Checkpoint | What it represents | What comparing it tells you |
|---|---|---|
| Sun | Core identity, vitality, life direction | Whether your basic ways of being energize or exhaust each other |
| Moon | Emotional needs, comfort, instinct | Whether you feel at home together when no one is performing |
| Venus | Affection, taste, how love is expressed | Whether your styles of giving and receiving warmth align |
| Mars | Desire, drive, how conflict is fought | Chemistry — and whether your tempers collide or coordinate |
| Ascendant | First impressions, outward manner | The surface fit: how naturally you read each other in daily life |
Work through them in order. Note the sign each placement occupies in both charts, then check whether any of your five touch any of theirs by aspect. Two passes — signs first, cross-aspects second — and you have already done more than most compatibility quizzes attempt.
The classic strong contacts
Certain cross-aspects earned a reputation over centuries of practice, and astrologers still check for them first.
- Sun-moon contacts. One person's sun aspecting the other's moon is the tradition's oldest marriage signature — a felt sense that one identity and one emotional nature fit together. Moon-sign compatibility between the two moons themselves is read as the quiet backbone of cohabitation.
- Venus-Mars contacts. The classical chemistry aspect: one person's style of loving meeting the other's style of wanting. Our guides to Venus in love and Mars and desire unpack each half.
- Ascendant contacts. A planet conjunct someone's ascendant tends to make its owner vivid to them immediately — the "I noticed you across the room" placement, in the tradition's telling.
Soft aspects (trines, sextiles) between these points are read as ease; conjunctions as intensity; squares and oppositions as heat that needs handling. All three categories can appear in the same pairing, and usually do.
Why one hard aspect never dooms a couple
Here is the mistake nearly every beginner makes: finding a Mars square Mars, or a Saturn opposition to a partner's Venus, and treating it as a diagnosis. The tradition itself never worked that way. A synastry chart contains dozens of contacts, and classical astrologers weighed the whole pattern — a single square inside a web of supportive aspects was read as a recurring argument, not a structural flaw.
There is a deeper point, too. Astrologers have long observed that charts with nothing but soft aspects describe pleasant, low-stakes connections — agreeable and sometimes forgettable. Friction aspects mark the places where two people actually have to negotiate, and negotiation is where intimacy gets built. If you want the psychological counterpart to that idea, the research tradition around conflict in couples — summarized in our piece on the Gottman four horsemen — makes a similar case: the presence of conflict predicts far less than the manner of it.
So when you find a hard aspect, the useful question is never "are we doomed?" It is "what recurring conversation does this describe, and how do we usually handle it?"
Beyond the first pass
Once the five checkpoints feel familiar, synastry opens outward. House overlays — where your planets land in your partner's houses — show which rooms of their life you occupy. Mercury contacts describe how you talk and misunderstand each other. Saturn contacts, in the tradition, mark where a bond feels serious, sometimes heavy, often durable. And when you are ready for a different lens entirely, composite charts blend two charts into a single chart of the relationship itself — a companion technique rather than a replacement.
None of this needs to arrive at once. A first synastry reading done honestly — five placements, a handful of cross-aspects, no catastrophizing — will tell you more than any percentage score.
Synastry rewards patience the way relationships do: the picture sharpens as you keep looking. If you want to start with the broad strokes before descending into aspects, our compatibility hub maps every sign pairing — and once you have both charts in hand, the aspects will do the rest of the talking.