Relationships
Synastry 101: How Astrology Reads Compatibility
"We're both Geminis, are we compatible?" is the question — but Sun-sign compatibility barely scratches the surface. Real astrological compatibility uses synastry: laying two complete birth charts over each other to see how each person's planets touch the other's. It's richer, more honest, and often surprising.
Compare two charts free. Our compatibility calculator overlays both charts, scores the match and explains the key connections.
What synastry actually looks at
Instead of one chart, synastry works with two — yours and theirs — and asks how they interact in three main ways:
1. Cross-chart aspects
The heart of synastry: the angles between your planets and their planets. Your Venus trine their Mars? Easy attraction. Your Saturn square their Sun? Lessons and friction. These cross-aspects describe the actual chemistry between two people.
2. Venus and Mars
Venus (how you love and what you value) and Mars (desire and drive) are the classic "attraction" planets. How your Venus and Mars connect with theirs says a lot about romance and spark.
3. House overlays
When you drop their planets into your houses, you see where they land in your life. Their Sun in your 7th house of partnership feels very different from their Sun in your 10th house of career. (Most astrologers use whole-sign or equal houses for overlays.)
Compatibility isn't a pass/fail score
A high "match score" is fun, but the real value is understanding how two people fit — where it flows and where it takes work. "Difficult" aspects like squares aren't dealbreakers; they're often the growth and tension that keep a relationship alive. Synastry describes the dynamic, not the destiny.
What you need to run it
For a full synastry reading you need both people's birth date, exact time and city. Without birth times you can still compare planets, but you'll lose the Ascendants and house overlays — the same rule as any birth chart reading.
See how two charts connect
Run a free comparison with the compatibility calculator — it draws both charts, finds the cross-aspects and gives an AI-written synastry reading. New to the basics? Start with rising signs and the zodiac guides.
Have a relationship or dating site? A branded compatibility widget can live on your own pages — see the astrology widgets.