What synastry is
Synastry compares two birth charts by measuring the aspects between one person's planets and the other's. Your Venus touching their Mars, your Moon meeting their Saturn, your Sun on their Ascendant – each contact describes a specific thread of attraction, comfort or friction running between you. It is far richer than "Sun sign compatibility," which compares only two of the twenty-plus placements involved. Synastry maps the dynamics of a bond, not a verdict on it: it shows where two people spark, soothe and snag, and leaves what they do with that entirely to them. It also applies to any relationship, not only romance – friendships, family ties and working partnerships all have synastry, and the same techniques read a business partnership as readily as a marriage. Anywhere two charts meet, there is a story in how they aspect each other.
The contacts that matter most
Some pairings carry outsized weight. Sun–Moon contacts speak to core compatibility, a meeting of identity and emotional need. Venus–Mars is the classic chemistry of attraction and desire. Moon–Moon shows whether two emotional rhythms naturally sync. Sun–Sun and Ascendant contacts govern how instantly two people "get" each other, while Saturn contacts bring duty, commitment and staying power – the glue that also grinds. The strongest bonds usually mix several of these, tying two charts together at more than one point rather than resting on a single dazzling aspect. Astrologers also watch the "luminaries and angles" – when one person's planet lands on the other's Sun, Moon, Ascendant or Descendant, the contact tends to be felt personally and instantly, far more than a link between two slow outer planets that any two people of the same generation would share.
Harmonious versus challenging links
As within one chart, the aspects split into easy and hard. Trines and sextiles between two people's planets feel smooth and supportive – you understand each other with little effort. Squares and oppositions bring charge, friction and growth, and, perhaps surprisingly, they are often what makes a relationship magnetic and alive. A bond built only on soft aspects can be pleasant but flat; one built only on hard aspects can be exciting but exhausting. The relationships that last usually blend both: enough ease to feel at home, enough tension to keep the current running. Astrologers pay special attention to "double-whammy" contacts, where each person aspects the other's planet – your Venus to their Mars and their Venus to yours – because a two-way link binds far more tightly than a one-way one, for better or for worse.
Saturn: the glue and the grind
Saturn deserves its own note, because it behaves differently from the rest. Strong Saturn contacts between two charts – Saturn on a partner's Sun, Moon or Venus – bring seriousness, structure and longevity; these are the aspects behind marriages and lifelong partnerships. But Saturn is also the planet of limits and duty, so the same contacts can feel heavy, critical or restrictive if the relationship turns sour. Saturn is what makes a bond durable and what can make it feel like work. Astrologers watch it closely precisely because it cuts both ways, and because a relationship with no Saturn contact at all can struggle to find the ballast to last.
Houses in synastry: where it lives
Beyond the aspects, synastry looks at which of your houses a partner's planets fall into – and this is often where a relationship's flavour truly shows. When their Sun lands in your 7th house, they feel like a natural partner; in your 5th, like romance and play; in your 10th, tied to your ambitions and public life; in your 4th, like home and family. The house overlay says which department of your life the other person naturally activates. A partner whose planets fall in your 12th house can feel fated but elusive; one whose planets fill your 3rd feels like easy daily conversation. It is one of the most revealing and underrated layers of the whole comparison, and it works both ways at once – you are landing in their houses just as they land in yours.
The composite chart
Comparing two charts shows how two people affect each other; the composite chart asks a different question – what is the relationship itself like? Astrologers build it from the midpoints of both charts, creating a single new chart treated as the entity the couple forms together, with its own Sun, Moon and Ascendant. Where synastry is two people in conversation, the composite is the third thing they create between them. A relationship can have warm synastry yet a difficult composite, or the reverse, which is why serious readings look at both. A common pattern: fiery synastry that feels magnetic between two people, sitting on top of a composite that struggles to build anything stable – great chemistry, hard to live in. The composite is where you ask not "how do we feel about each other?" but "what does this relationship, as a thing, want to become?"
The Davison chart
There is a second way to chart a relationship as one entity: the Davison, or time-space midpoint, chart. Rather than averaging chart positions like the composite, it is cast for the exact midpoint in time between the two births and the geographic midpoint between the two birthplaces – so it produces a real date, time and place, and therefore a chart that can itself be transited and progressed. Astrologers who want to track how a relationship evolves over the years often reach for the Davison alongside the composite. Both treat the couple as a living thing with a chart of its own. The practical difference: because the Davison has a real date and place, you can run transits to it and watch the relationship pass through its own seasons, something the purely averaged composite does less naturally. Which you prefer is partly a matter of school and taste; many astrologers glance at both and trust what the two agree on.
What synastry can and cannot tell you
Synastry is a language for understanding a relationship, not a scoreboard for it. It can show where two people flow and where they friction, why a certain dynamic keeps repeating, and what each person tends to bring out in the other. It cannot deliver a compatibility percentage that decides whether you belong together, name a "soulmate," or predict whether you will last – no chart contact overrides two people's choices, effort and timing. Difficult synastry has carried happy marriages; perfect synastry has ended in weeks. Its real gift is language: it gives a couple words for a pattern they already feel – why one keeps chasing and the other withdrawing, why a certain subject always sparks – and naming a dynamic is often the first step to handling it well. Read it as insight, never as a sentence.
How AstraBrain reads compatibility – honestly
AstraBrain computes both people's charts with the Swiss Ephemeris and analyses the synastry between them – the exact aspects, the house overlays, the standout contacts – for its compatibility readings. That mathematics is precise astronomy, accurate to the arc-minute. The interpretation of what those contacts mean for the two of you is written by AI, offered as a lens for understanding a relationship, never as a final judgement on it and never as prediction. We measure how two charts meet exactly; what you make of it is, as always, yours.
Quick questions
Are some signs simply incompatible? No – every pairing has workable and challenging contacts; the whole chart matters far more than two Sun signs. • Do we both need birth times? For the full picture yes – the Moon, Ascendant and houses need them – but a partial reading is still possible without. • What makes a "good" match astrologically? Usually a blend: enough easy contacts to feel at home and enough Saturn and challenge to give it depth and staying power. • Can synastry predict a breakup? No – it shows dynamics and pressures, not outcomes; how a couple handles them decides the ending, not the chart.