What Is a Synastry Chart? How Astrologers Analyze Compatibility

A synastry chart is an astrological technique that places two people's birth charts side by side, or overlays them, to analyze how the planetary positions of one person interact with the planetary positions of the other. The result is a map of the relationship itself: where the two of you naturally harmonize, where friction lives, and what drives the pull between you.

Unlike a single natal chart, synastry is inherently relational. It doesn't describe either person in isolation. It describes the dynamic that emerges when those two individuals occupy the same space and life.

How Synastry Works

Every person has a natal chart: a snapshot of where every planet in the solar system was positioned at the exact moment of their birth, from the perspective of their birthplace. That chart includes the Sun, Moon, Mercury, Venus, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto, each in a sign, in a house, and in geometric relationship to other planets (called aspects).

Synastry takes Person A's chart and Person B's chart and looks at how those two sets of planets interact. If Person A's Venus is at 14° Taurus and Person B's Mars is at 12° Taurus, those two planets are conjunct, meaning they're occupying nearly the same degree of the zodiac. That conjunction between one person's Venus and another's Mars is a classic indicator of attraction.

The planets don't merge into each other's chart; they remain distinct. What synastry analyzes is the conversation happening between them.

Astrologers pay particular attention to inter-aspects (aspects formed between charts), which planets are involved, and which houses each planet falls into in the other person's chart.

Venus-Mars Aspects: The Chemistry

When astrologers talk about chemistry, the immediate, almost inexplicable pull between two people, they're often looking at the Venus-Mars axis.

Venus represents how we attract, what we find beautiful, and how we approach love and pleasure. Mars represents how we pursue, assert, and express desire. When one person's Venus makes a significant aspect to the other person's Mars, that interaction tends to produce a noticeable charge.

The conjunction (same degree) is the most intense form: the two planets are merged, and attraction can feel almost fated. The trine (120°) creates a flowing ease; flirtation comes naturally, there's a comfort to being desired by this person. The sextile (60°) adds a playful, collaborative dimension.

Squares (90°) and oppositions (180°) between Venus and Mars are often misread as bad signs. In practice, they frequently indicate strong attraction alongside friction, the kind of tension that keeps things interesting, but also requires conscious management. A square can feel electric and frustrating in equal measure.

Connections between both Venus placements, or both Mars placements, also matter. Venus-Venus trines create a natural aesthetic alignment; you tend to want the same kind of evening out. Mars-Mars squares can produce competition or mutual motivation, depending on how self-aware both people are.

Sun-Moon Connections: Emotional Compatibility

If Venus-Mars is about chemistry, Sun-Moon aspects are about compatibility in a deeper sense, the ability to actually live alongside someone without feeling fundamentally at odds.

The Sun represents identity, ego, and the conscious sense of self. The Moon represents emotional needs, instincts, and how we feel at home. When one person's Sun aspects another's Moon, something important happens: one person's core identity meets the other's emotional core.

A Sun-Moon conjunction across charts, where your Sun sits near my Moon, or vice versa, is one of the most significant indicators of deep personal compatibility in astrology. There's a quality of natural understanding, a sense that the other person gets you at a level that doesn't require explanation.

Sun-Moon trines create a similar ease without the intensity. Sun-Moon squares and oppositions can produce a push-pull: you feel drawn to each other, but there's an ongoing negotiation between what one person needs for their sense of self and what the other needs emotionally.

Mutual Sun-Moon contacts, where both people's Suns aspect each other's Moons, are comparatively rare and tend to produce relationships of real depth and staying power.

House Overlays: Where You Activate Each Other

Beyond aspects, synastry examines where each person's planets fall in the other's house system. This is called the house overlay.

If your Sun falls in my 7th house (the house of partnerships), you may represent exactly what I'm looking for in a partner, almost the projection of my ideal. If your Saturn falls in my 4th house (home and family), there may be a sobering, structuring effect on domestic life, stabilizing or constraining depending on context.

Certain house overlays are particularly significant:

• 5th house overlays (creativity, romance, play): planets placed here create a sense of joy and romantic excitement
• 7th house overlays (partnership): the person feels like a natural partner archetype
• 8th house overlays (depth, transformation, shared resources): intense, transformative, sometimes uncomfortable
• 12th house overlays (the unconscious, hidden matters): profoundly intimate and sometimes mysterious; the other person sees parts of you that are hard to access consciously

House overlays don't override aspects, but they add context. A Venus overlay in someone's 5th house explains why a Venus-Mars square still reads as wildly attractive rather than just aggravating.

The Composite Chart: Your Relationship's Own Identity

Synastry describes the interaction between two natal charts. The composite chart is a distinct technique that takes the midpoints of both people's planets to create a third chart, one that represents the relationship itself as an entity.

Where synastry says "here's how your Venus talks to my Mars," the composite chart says "here's what Venus and Mars mean for the relationship you've created together."

Composite charts are useful for understanding the long-term character of a relationship: its purpose, its patterns, and the evolutionary arc it seems to want to take. A composite Sun in Capricorn in the 10th house suggests a relationship with strong ambitions and a public dimension. A composite Moon in Cancer in the 4th house suggests a relationship that revolves around home, family, and emotional security.

The two techniques are complementary, not competing. Synastry shows the chemistry; the composite shows the character of what you build together.

What Makes a "Good" Synastry?

There's no synastry chart that guarantees a happy relationship, and no synastry chart that predicts disaster. What astrologers look for is pattern, not perfection.

Helpful indicators: mutual Sun-Moon contacts, Venus-Mars trines or conjunctions, benefic planet overlays in partnership and romance houses, and a general balance between supportive and challenging aspects.

Challenging indicators to work with: Saturn contacts (which bring structure but also restriction), Pluto contacts (which intensify and can destabilize), and heavy 12th house overlays (which can create a hidden or confused quality).

The most honest answer is that strong synastry between two emotionally mature people tends to produce strong relationships. Strong synastry between two people who lack self-awareness tends to produce strong drama.

Synastry shows the terrain. What you do with it is still up to you.

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If you want a full synastry analysis for you and a partner, including all inter-aspects, house overlays, and composite chart interpretation, see the Western Compatibility Report. It covers everything explored here in depth, specific to your two birth charts.