Synastry Explained: Astrological Compatibility Basics

Two people meet. Something clicks — or doesn't. Astrology has spent millennia trying to explain why.

Synastry is the branch of Western astrology that compares two birth charts directly, planet by planet, to map the dynamics between two people. It is the most technically rigorous tool astrology offers for relationship analysis. It is also one of the most misused, reduced to "your Venus sign matches his Sun sign, you're perfect," which is roughly as useful as saying two people are compatible because they both like coffee.

This guide covers the actual mechanics: what synastry measures, how to read it, where it earns its credibility, and how traditions outside Western astrology — BaZi, numerology, I Ching — approach the same question from different angles.

What synastry actually is

Synastry overlays one person's natal chart on top of another's. You are not creating a new chart — you are reading how Person A's planets interact with Person B's planets, and vice versa.

Every planet in your chart occupies a sign and a degree. When you compare charts, you measure the angular distances — called aspects — between the planets. A Sun at 15° Aries and a Moon at 17° Libra are roughly 180° apart: an opposition. A Venus at 10° Taurus and a Mars at 12° Cancer are roughly 60° apart: a sextile.

Those angles carry meaning. The question is which planets are involved and what the aspect between them suggests about the dynamic.

The planets that matter most in synastry

Not every planetary contact carries equal weight. In practice, practitioners work a rough hierarchy.

Tier 1 covers core identity and connection: the Sun (ego, identity, life direction), the Moon (emotional needs, comfort, instinctive reactions), Venus (what you find beautiful, how you love, what you attract), Mars (drive, desire, how you pursue and conflict), and the Ascendant (how you present, first impressions, physical response).

Tier 2 is about depth and longevity: Saturn (commitment, structure, longing combined with restriction), Jupiter (expansion, generosity, where one person opens the world for the other), and Chiron (wounds and healing, where one person can either hurt or profoundly help the other).

Tier 3 — Pluto, Neptune, Uranus — moves slowly. Contacts here tend to describe generational dynamics or unconscious undercurrents rather than anything you'd notice on a Tuesday.

The major aspects and what they signal

Aspect	Angle	Orb (synastry)	Quality
Conjunction	0°	±8°	Intense fusion — can be magnetic or overwhelming
Opposition	180°	±8°	Polarity — attraction through difference, potential friction
Trine	120°	±8°	Ease, flow, natural harmony
Square	90°	±7°	Tension, friction, growth through challenge
Sextile	60°	±6°	Opportunity, mild support
Quincunx	150°	±3°	Awkwardness, adjustment required

A relationship built entirely on trines and sextiles often feels pleasant but can go soft — no friction, no real drive. Squares and oppositions generate chemistry and tension, the kind that makes a relationship feel alive and also difficult. The most durable relationships in astrological literature tend to have a mix: some ease, some challenge, and strong Saturn contacts that provide the container.

House overlays: where someone lives in your life

Beyond aspects, house overlays matter enormously and beginners consistently underweight them.

When Person A's Sun falls into Person B's 7th house, Person B instinctively reads Person A as a partner figure. If Person A's Mars falls in Person B's 12th house, Person A's drive and assertiveness might feel threatening or confusing to Person B — not because either is doing anything wrong, but because the 12th house is where things go murky.

Key overlays to examine:

• 1st house: strong physical and identity resonance
• 4th house: domestic, family-like feeling; can deepen roots
• 5th house: romance, play, creative spark
• 7th house: partnership energy, seeing the other as "the one"
• 8th house: intensity, transformation, intimacy — and power dynamics
• 12th house: spiritual connection or hidden tension, often both

A worked example

Take two fictional charts:

Person A: Sun at 14° Scorpio, Moon at 22° Pisces, Venus at 3° Libra, Mars at 18° Leo

Person B: Sun at 16° Taurus, Moon at 20° Virgo, Venus at 5° Sagittarius, Mars at 21° Pisces

What immediately stands out:

1. A's Sun opposite B's Sun (Scorpio-Taurus, 14° vs 16° — a 2° orb opposition). Classic polarity. These two signs share fixed-sign stubbornness and an orientation around desire, resources, and depth. The opposition creates pull. They will likely find each other compelling and maddening in equal measure.

2. A's Moon opposite B's Moon (Pisces-Virgo at 22° and 20° — Pisces and Virgo are 180° apart, so this is a Moon opposition Moon). Emotional needs differ sharply; Pisces Moon needs fluidity and emotional merging, Virgo Moon needs order and practicality. They can learn from each other or exhaust each other.

3. A's Mars trine B's Mars (18° Leo vs 21° Pisces — roughly 123° apart, close to a trine). A Leo Mars pursues with flair and confidence; a Pisces Mars pursues indirectly, emotionally. The trine means their drives broadly support each other without direct competition.

4. A's Venus sextile B's Venus (Libra 3° vs Sagittarius 5° — 62°, a sextile). Their aesthetic sensibilities and styles of affection complement each other without being identical. Easy, not electric.

This is not a perfect pairing or a doomed one. It is a specific map — where there will be magnetic tension, where there will be emotional friction, where there will be ease.

What synastry cannot tell you

Synastry reads potential dynamics. It does not tell you:

• Whether either person is emotionally available
• Whether the timing is right (that is the domain of transits and progressions — see our article on how eclipse seasons affect your chart)
• Whether both people have the self-awareness to work with their chart dynamics rather than be driven by them
• Whether the relationship is romantic, familial, or professional — all of these can produce identical synastry patterns

A strong Saturn contact — one person's Saturn conjunct the other's Moon, for instance — appears in long-term marriages and in relationships people describe as oppressive. Same planet, same aspect. The human context determines which way it runs.

The composite chart: a different tool

Synastry looks at two charts in relation to each other. The composite chart takes the midpoints of both charts — the midpoint of both Suns becomes the composite Sun, and so on — and creates a single chart representing the relationship itself as an entity.

Where synastry asks "how does A affect B and vice versa?", the composite chart asks "what is this relationship as a third thing?"

A composite Sun in the 10th house suggests a relationship with a public dimension, partnerships that build something visible in the world. A composite Moon in the 4th suggests the emotional core is private, home-based, family-oriented.

Serious practitioners use both: synastry for interpersonal dynamics, composite for the relationship's character and purpose.

How BaZi reads compatibility

Western synastry is not the only rigorous framework for relationship analysis. BaZi — the Chinese Four Pillars system — approaches compatibility through elemental interaction between two people's charts.

In BaZi, each person's chart is built from four pillars (Year, Month, Day, Hour), each containing a Heavenly Stem and Earthly Branch. The Day Stem is considered the Day Master — the core of who you are. Compatibility analysis examines how the Day Masters and Branches of two charts interact according to the Five Element cycles: the generating cycle (Wood feeds Fire, Fire creates Earth, Earth produces Metal, Metal carries Water, Water nourishes Wood) and the controlling cycle (Water quenches Fire, Fire melts Metal, and so on).

A Wood Day Master paired with a Water Day Master sits in a generating relationship — Water feeds Wood. That suggests support and nourishment. A Wood Day Master paired with a Metal Day Master sits in a controlling relationship — Metal chops Wood. That can mean friction or, depending on the chart's balance, the kind of productive pressure that actually shapes someone.

BaZi also looks at Branch combinations and clashes — specific pairings of Earthly Branches that produce harmony, transformation, or conflict. A Rat-Ox combination (子丑合) is one of the six harmonious combinations, associated with partnership energy. A Rat-Horse clash (子午冲) introduces instability and change.

For a deeper look at BaZi mechanics, the BaZi guide on Zodiacs covers the framework in full.

Numerology and relationship numbers

Numerology takes the most stripped-down approach: reduce the birth dates to Life Path numbers and examine their relationship.

Life Path numbers run 1-9 (with master numbers 11, 22, and 33 as special cases). The traditional compatibility logic works on resonance and complementarity:

• Life Path 2 (partnership, sensitivity) is frequently cited as compatible with Life Path 8 (ambition, authority) — the 2 provides the relational support the 8 needs; the 8 provides the direction the 2 wants.
• Life Path 4 (structure, reliability) and Life Path 7 (introspection, depth) — less obviously compatible on the surface, but both value seriousness and have no patience for superficiality.
• Same Life Path numbers can produce deep resonance or fierce competition, depending on the maturity of both people.

Numerology compatibility is the least granular of these systems — it works with a single number per person, losing enormous amounts of birth chart detail. Treat it as a starting prompt, not a conclusion. Our numerology Life Path guide covers the calculation in full.

I Ching and the question frame

The I Ching approaches relationship questions differently from the other systems — it does not profile two people against each other. Instead, it responds to a specific question asked at a specific moment.

"What is the nature of this connection?" or "What should I understand about this relationship right now?" — cast a hexagram, read the moving lines, follow the transformation hexagram if lines are present.

Hexagram 31 (Xián / Influence) is the classical I Ching hexagram associated with courtship and mutual attraction — Lake over Mountain, the yielding attracting the receptive. Hexagram 37 (Jiā Rén / The Family) addresses established relational structures. But the I Ching does not hand you a compatibility score. It hands you a frame for reflection at a particular moment. That is a different kind of usefulness — and an honest one.

Putting it together

No single system covers everything. Western synastry gives you the most granular interpersonal map — which planets are in tension, which support each other, where the friction lives. BaZi situates both people within elemental dynamics and timing cycles. Numerology offers a fast structural note. I Ching responds to a live question with a live answer.

Used together, they triangulate rather than duplicate. A synastry chart showing strong Moon-Saturn contacts between two people becomes more legible when you also know their BaZi charts are in a controlling element relationship — both traditions pointing at the same underlying pattern of structure, pressure, and potential restriction. That convergence is worth paying attention to.

If you want to run your own chart and see how these layers interact, open Zodiacs and pull your reading.

Frequently Asked Questions

What is the most important aspect in synastry?

There is no single most important aspect — it depends on the pair. That said, Sun-Moon contacts (especially conjunctions and oppositions), Venus-Mars contacts, and any strong Saturn aspects to personal planets are among the most consistently significant. Sun-Moon conjunctions between two people are a classic marker of deep personal resonance; Saturn contacts to Venus or the Moon often describe long-term commitment with a side of emotional weight.

Can synastry predict whether a relationship will last?

No. Synastry maps dynamics, not outcomes. A chart full of challenging aspects between two emotionally intelligent, committed people can describe a relationship that lasts decades. A chart full of harmonious aspects between two people who are unavailable or immature can dissolve in months. Synastry shows you the terrain; it does not walk the path for you.

Do you need exact birth times for synastry?

For house overlays and Ascendant contacts, yes — without a birth time you cannot calculate houses accurately. For planet-to-planet aspects using the Sun, Moon (roughly, within a day), Mercury, Venus, Mars, Jupiter, and Saturn, you can still do meaningful work without an exact time, though the Moon moves about 12-13° per day, so a birth time error of a few hours can shift the Moon's degree significantly.

How is BaZi compatibility different from Western synastry?

Western synastry works with the geometric relationships between planets in two charts — aspects and house overlays. BaZi compatibility works with elemental interactions between two people's Day Masters and the Branches in their pillars. BaZi is less focused on the psychological texture of specific relationship dynamics ("your Mars squares my Venus means X") and more focused on whether the elemental balance of two charts supports or drains each other, and whether specific pillar interactions create harmony or conflict. They are measuring related but distinct things. For a fuller comparison of the two systems, the BaZi section covers the framework in depth.

Is Sun sign compatibility (e.g., Scorpio and Taurus are compatible) reliable?

It captures something real but with very low resolution. Sun signs describe a slice of identity and life orientation — two Scorpio-Taurus pairings might share a polarity dynamic, but one might involve a Scorpio with a Pisces Moon and Libra Rising and the other a Scorpio with an Aries Moon and Capricorn Rising. These are profoundly different people despite sharing a Sun sign. Sun sign compatibility is a starting sketch, not a reading.

What is the difference between synastry and a composite chart?

Synastry compares two charts directly — A's planets interacting with B's planets. A composite chart takes the mathematical midpoints of both charts to create a single chart representing the relationship as its own entity. Synastry tells you how two people affect each other. The composite tells you what the relationship itself is — its purpose, character, and challenges as a shared unit. Both are useful; serious relationship readings typically use both.