What Is Synastry? The Meaning of Relationship Astrology
Synastry is the branch of astrology that compares two birth charts side by side to understand a relationship — romantic, family, or otherwise. The term comes from the Greek for "stars aligned together," and the practice is older than most modern astrology you'll encounter.
Where sun-sign compatibility flattens everyone into twelve types, synastry treats each person as a full chart and asks how those two charts interact.
What Is Synastry?
Synastry is the astrological practice of placing two birth charts on the same wheel and reading how the planets, signs, and houses of one chart interact with those of the other. It's sometimes called relationship astrology.
The goal isn't to predict whether a relationship will work. It's to understand the dynamics already at play — what flows easily, what creates friction, what each person brings to the connection. Synastry describes patterns. It doesn't issue verdicts.
How Synastry Works: Comparing Two Birth Charts
A synastry reading starts with two complete birth charts — one for each person. Each chart needs date, time, and place of birth to be accurate.
Once both charts are drawn, an astrologer looks at three things:
- Where each person's planets fall in the other's chart — for example, your Venus landing in their 7th house
- Aspects between the two charts — angles between planets in one chart and planets in the other, like one person's Mars conjunct the other's Moon
- Shared placements — when both people have planets in the same signs or houses
The combination of these three layers shows the texture of the relationship. A strong sun-sign match can sit alongside difficult Venus-Mars aspects, or vice versa.
To compare two charts, you need both sets of birth data and a tool that overlays them. Use our free synastry chart to generate one.
The Big Three in Synastry: Sun, Moon, and Rising
The "big three" in synastry are the same as in a single birth chart: sun, moon, and rising sign. They're the most important placements to compare first because they describe core identity (sun), emotional life (moon), and outward presentation (rising).
Strong contact between two people's big three — same signs, same elements, or harmonious aspects — usually shows up as a sense of recognition. The people feel like they "get" each other quickly. Difficult contacts between the big three don't doom a relationship, but they often mean the connection takes more translation work, especially early on.
For romantic synastry, Venus and Mars matter almost as much. Venus describes what you love and how you give affection. Mars describes desire and how you pursue what you want. The interaction between two people's Venus and Mars is often where physical chemistry shows up most clearly.
Houses That Matter Most in Synastry
Of the twelve houses in astrology, three carry extra weight in relationship readings:
- The 5th house — pleasure, romance, creativity, and dating. Planets falling here in a partner's chart often signal attraction and play.
- The 7th house — partnership, marriage, and one-to-one relationships. Heavy 7th-house activity between two charts often points to long-term commitment energy.
- The 8th house — intimacy, shared resources, and deeper psychological merging. The 8th house is where relationships move past surface compatibility into transformation.
Other houses still play a role — the 4th for home-building, the 11th for friendship — but the 5th, 7th, and 8th are where most synastry analysis begins.
Synastry vs Composite Chart: What's the Difference?
Synastry and composite charts are both used to understand relationships, but they show different things.
A synastry chart keeps both people's charts intact and reads them side by side. You see two distinct individuals and how they meet. A composite chart, by contrast, takes the midpoints between each pair of planets and creates a single new chart — a chart of the relationship itself, as if the relationship were its own entity.
Most astrologers use both. Synastry tells you what each person brings. The composite tells you what they become together.