What is synastry (chart compatibility)?

What is synastry?

Synastry is the branch of astrology that compares two people’s birth charts to examine the chemistry of their relationship. It doesn’t hand out a single "compatible/incompatible" label; it shows where the relationship flows easily and where there is friction and growth.

This is far more detailed than Sun-sign compatibility. Two Aries can be "incompatible," or an Aries-Cancer pair can form a deep bond — because what matters is not just the Sun, but all the aspects the two charts make to each other.

Aspect overlay: two charts on top of each other

The core technique of synastry is placing one person’s planets over the other’s chart and measuring the aspects between them. If your Venus trines their Mars, attraction flows; if your Saturn squares their Moon, there may be an emotional responsibility-restriction tension.

It also matters which houses one person’s planets fall into in the other’s chart: if their Sun falls in your 7th house, that person awakens the "partner/spouse" archetype in you.

The most meaningful connections

Some contacts are especially strong in romantic relationships: Sun-Moon (the meeting of identity and emotion), Venus-Mars (attraction and desire), Moon-Moon (emotional rhythm alignment), and Moon-Ascendant (a sense of comfort and familiarity). Saturn contacts make the bond serious but can weigh it down; outer-planet (Uranus, Neptune, Pluto) contacts can be intense yet challenging.

Even a single tight aspect can set the tone of a relationship. What matters is not an abundance of aspects, but which planets touch and in what tone (soft or hard).

How it differs from the composite chart

Synastry looks at the interaction of two separate charts; the composite chart creates a single "relationship chart" from the midpoints of the two. Synastry answers "what are you doing to me," the composite answers "who are we together." The two complement each other.

Beyond Sun-sign compatibility

Sun-sign compatibility tables are a fun starting point but stay superficial. Real compatibility looks at how the whole charts — especially the Moon, Venus, Mars, and angular points — overlap. That’s why two different relationships from the same sign pair can feel completely different.