How to read a synastry chart: Introduction to relationship compatibility
- Julia Topaz
- Sep 4, 2017
- 8 min read
Updated: May 22
What is a synastry chart?
A synastry chart is one of astrology’s most powerful tools for understanding how two people affect each other. While a birth chart reveals your inner wiring, your emotional needs, patterns, desires, a synastry chart overlays one chart onto another to explore relational chemistry, compatibility, and karmic themes.
By comparing the positions of the planets, aspects, and house overlays, a synastry chart shows how your energies interact. It reveals why you’re drawn to someone, what triggers you, and whether your connection nurtures growth or recreates past wounds.
It’s not just about romantic compatibility. Synastry applies to friendships, family dynamics, and business partnerships, any bond where emotional entanglement occurs. It helps you name the patterns you keep repeating and understand whether they come from connection, codependency, or karma.
In this guide, we’ll walk through the key things to look for in synastry: from emotional patterns to red flags, from soulmate potential to toxic entanglement, so you can approach relationship astrology with clarity and depth.
And if you’re new to astrology, don’t worry, we’ll start with the basics and build up. See all of our posts on synastry
How to read a synastry chart?
If you’re a beginner in Astrology, reading a synastry can be a challenging exercise. In this post we will cover a non-exhaustive list of things to look for when looking at a synastry for the first time. Once you read this introduction, you can read our other posts on synastry.
How to pull a synastry chart?
To pull a synastry chart, you need to use either an app or a website. I like to use this site. You'll need to enter the date, time and location of birth for both partners and pull their respective birth charts. Once you've done this, head to "extended charts" and in the chart selection, select "synastry with house overlays"
What if you don't have your partner's time of birth?
You can still do a synastry chart if you do not have their time of birth. In this case I suggest doing the following: 1. Pull your partner's chart at 00:01am and 11:59pm. Note the range of the Moon on that day and see if it changes signs or stays in the same zodiac sign for the entire day. This will allow you to possibly refine your partner's time of birth possibly based on their personality traits.
Pull the synastry chart but don't include "house overlays". You'll be able to determine where their planets fall in your chart, but not the other way around.
Step 1: Understanding each birth chart separately
Before diving into comparison, you must understand the birth chart of each person.
What emotional needs do they have?
How do they express love?
What kind of relationships do they tend to attract?
What are things they personally struggle with: Trust? Insecurity? Impulsivity?
You can use our free birth chart tool to generate your placements along with a free interpretation of your natal chart. This step is so important because we must understand each person as independent entities before interpreting how they come together.
I actually start every synastry reading by first doing a quick run down of each partner's birth chart. For example if you're someone who has a strong Pluto, you'll really struggle being with someone who doesn't have strong Pluto energy. If that intensity isn’t matched, frustration builds.
This step matters because:
A difficult synastry can still serve your growth, depending on your natal blueprint.
Great synastry doesn’t equal emotional health—you can feel magnetic chemistry with someone completely emotionally unavailable.
Zodiac sign incompatibility doesn’t always hold up. A Libra and Capricorn might seem like opposites, but if the Capricorn has planets in Libra or the 7th house, the dynamic might work surprisingly well. You do need to understand the basics of birth chart reading to understand meaningfully a synastry. If you need help with this, you can take our introduction crash course. Not everyone is cut out for the same type of relationship. Someone with an easy-flowing natal chart may run away from a dramatic relationship. Someone with challenging aspects in their own natal chart tend to be used to conflicts. We usually look for what we know and what we're familiar with, so you may be looking for someone who reproduces the conflictual energies you're used to.
Not all squares are equal. Saturn square Moon in synastry for example is always a difficult aspect to have a synastry no matter how you put it. It might not be a dealbreaker but it is tough. Mars square Venus is entirely less difficult.
If there are hard aspects in the synastry from the outer planets, you do want to be mindful that this is a healthy relationship and not a toxic relationship. Especially if you find strong aspects of Pluto such as Pluto-Mars in the synastry chart, you want to be sure you're both in the relationship for the right reasons.
Step 2: The big picture of the synastry
Zoom out and look at the big picture:
What’s the most aspected planet? For example if it's Mars, it means hot hot but also possibly conflict. If it's Moon, it might be a particularly emotionally attuned relationship.
Are any planets unaspected or isolated? This can show you what is lacking possibly in that specific relationship.
Does one chart dominate in fire, earth, air, or water? Are there any glaring imbalances?
Are there similar patterns? For example, both partners have Moon in aspect to Neptune?
For example, if one person’s Moon receives no aspects, that person may feel emotionally unseen in the relationship. If one partner’s Saturn is heavily involved, there might be themes of restriction, duty, or karmic lessons.
Pay attention to Venus and Mars for chemistry, and Sun/Moon aspects for emotional core compatibility.
See also: Chemistry VS Compatibility
Step 3: Interpreting the aspects between planets in synastry
Planet | Affects | Keyword (What it Does) |
Sun | Identity, ego, willpower | Shine |
Moon | Emotions, instincts, needs | Feel |
Mercury | Communication, thoughts, logic | Think |
Venus | Love, beauty, values, affection | Value |
Mars | Desire, anger, sex, action | Pursue |
Jupiter | Growth, belief, expansion | Trust |
Saturn | Authority, structure, fear | Restrict |
Uranus | Change, freedom, disruption | Shock |
Neptune | Illusion, dreams, ideals | Dissolve |
Pluto | Power, transformation, obsession | Expose |
Chiron | Wounds, healing, sensitivity | Wound or Heal |
🔑 Aspect Keywords (Energy Expression)
Aspect | Keyword |
Conjunction | Merge |
Sextile | Encourage |
Trine | Flow |
Square | Challenge |
Opposition | Polarize |
In synastry, inter-aspects refer to the dynamic connections formed when one person’s planet makes an aspect to another person’s planet. These aspects reveal how two psyches interact, how one person impacts, activates, challenges, or supports the other.
The most important thing to understand is that the planet of Person A influences the function of the planet of Person B. For example, if your Mars aspects someone’s Venus, your desire awakens their sense of attraction. If your Saturn aspects their Moon, your boundaries, fears, or commitment style affect their emotional world. Each aspect (conjunction, trine, square, sextile, opposition) changes the flavor of this interaction—from ease and support to tension and resistance. Together, inter-aspects form the emotional, sexual, and psychological glue of the relationship, or the cracks in its foundation.
You want to look at what is called inter-aspects between the planets. The overall way this works is the planet person A impacts the planet person B. You can use the keywords I added above to understand how this works.
Step 4: Look for double whammies
Look for what we call “double whammies” in the synastry chart. It’s a double aspect, for example, your Mars trine the person’s Pluto, and the person’s Mars conjunct your Pluto. Energies are going both ways, there’s a pattern here.
Step 5: House Overlays
In synastry, house overlays show which area of life your partner activates in you and vice versa. While planetary aspects reveal the quality of your connection, house overlays tell you where the energy plays out.
When you place one person’s planets into the other person’s chart, you see which houses those planets fall into. This gives huge insight into how you experience them, what roles they unconsciously take on in your life, and where the emotional charge gets activated.
Think of it this way:
Your planets in their houses = how you impact them
Their planets in your houses = how they impact you
Each house governs a domain of life. When someone’s Venus falls in your 10th house, it feels very different than if it falls in your 4th.
Here’s a simplified breakdown:
1st house: You see them as attractive or familiar. Chemistry is instant. You feel "seen."
2nd house: They influence your values, money, and self-worth—for better or worse.
3rd house: A mental spark. They stimulate your mind, but the connection may stay intellectual.
4th house: Deep emotional safety or triggering. They touch your roots, your past, your inner child.
5th house: Attraction, passion, flirting. They make you feel alive. Sex and creativity flow.
6th house: Practical love or power imbalance. They may feel helpful or controlling.
7th house: Classic partnership energy. You may see them as “the one.” There’s projection here.
8th house: Deep karmic ties, sexual obsession, shadow work. Not for the faint of heart.
9th house: They expand your worldview, beliefs, or inspire you to grow.
10th house: You admire them or feel judged by them. Public image is involved.
11th house: Friendship, shared dreams, community.
12th house: Psychic bond or total confusion. Feels fated, spiritual, or lost in illusion.
Why it matters:
Someone could have "good" aspects with you—but if all their planets fall in your 6th or 12th house, you might not feel seen, understood, or emotionally fulfilled. On the flip side, even challenging aspects can feel meaningful if they hit your 4th, 7th, or 8th houses.
House overlays explain why someone feels important to you, even when the relationship doesn’t make logical sense. They show what part of your life they're woven into.
See also: House Overlays in Synastry
Step 6: Look at the composite chart
If you find the synastry chart confusing or inconclusive, the composite chart often brings clarity. You may see themes echoed here that weren’t as obvious in the synastry. For example, if your relationship feels emotionally ungrounded but you can’t explain why, a composite Moon square Neptune might explain the emotional fog.
While synastry focuses on how two separate charts interact, the composite chart represents the relationship itself as a third entity, a symbolic “child” of both charts.
A composite chart is calculated by finding the mathematical midpoints between each person’s planetary positions. For example, if one person’s Moon is at 10° Cancer and the other’s is at 10° Virgo, the composite Moon will fall at 10° Leo. This midpoint method gives birth to a new chart that describes how the relationship feels from the inside.
In simpler terms:
Synastry = how you affect each other
Composite = what the relationship feels like when you’re together
This is why composite charts are particularly helpful for long-term relationships or relationships with history. They reveal the purpose, core dynamic, and shared experience of being in the relationship.
The composite Sun shows the core identity of the relationship: why it exists and what it's here to achieve.
The composite Moon speaks to the emotional rhythm and security the relationship offers.
Composite Venus and Mars reflect the romantic and sexual dynamic.
Composite Saturn is a big one—it shows whether the relationship has staying power or feels heavy and burdened. Saturn contacts can bring longevity, but also tests and restrictions.
Outer planets like Neptune and Pluto can reveal deep spiritual connection—or confusion, illusions, and power struggles.
You can also analyze the houses where planets fall. For example, a composite Sun in the 7th house points to a relationship centered around partnership and shared identity. A 10th house composite Moon might indicate a public or status-oriented bond, where the relationship’s emotional expression is linked to career or visibility.
A synastry chart doesn’t dictate your fate, but it does provide a map of your energetic chemistry with someone else. It helps you understand:
Why you feel instantly drawn to someone
Why certain patterns repeat
Whether your relationship meets your core needs
By combining birth charts, natal charts, house overlays, and the composite chart, you can get a full picture of your relationship’s potential—and its challenges.
Awesome guide! Thank you so much!!!