The question everyone brings
Almost everyone who asks me to compare two charts wants the same thing underneath the words. They want a verdict. Does this work. Are we right for each other. Should I stay.
I understand the wish. It would be a relief if the sky handed out a yes or a no. What the technique actually does is quieter and, in the end, more helpful. It does not grade a relationship. It describes one. It shows you the shape of what happens when these two particular people are in a room together, where the warmth pools and where the friction catches. Then it leaves the deciding to you.
The name for it is synastry. It has been part of the craft for a very long time. Comparing two charts is one of the older applications of astrology, not a modern invention.
What synastry is, mechanically
You start with two complete birth charts, one for each person, each drawn for their own date, time and place. Then you lay them over each other and look at how the planets of one land on the planets of the other.
The main thing you are reading is the aspects between the two charts. An aspect is a meaningful angle between two points. Here the two points belong to different people. When one person's Venus, the planet of affection and taste, sits at the same degree as the other person's Mars, the planet of drive and desire, that is a cross-chart aspect. It describes a specific flavour of attraction between them. Multiply that by every planet meeting every other planet and you get a map of how two people affect each other.
There is a second layer underneath the aspects. Each person's planets fall into the houses of the other person's chart, the twelve areas of life. My Sun might land in your house of home. Or your house of shared resources. Or the house of your daily work. That placement tends to colour where in life I show up most vividly for you. The houses add the where to the how.
Not every cross-chart connection matters equally. A handful of contacts tend to do most of the describing.
Sun-to-Sun and Sun-to-Moon contacts speak to the basic ease or effort of being around each other, whether your core rhythms settle or grate. Moon to Moon reaches into the emotional register, how safe each person feels, whether the unspoken needs line up or pull against each other. Venus and Mars between the charts tend to describe desire and the texture of attraction, the difference between a warm steady liking and a charged pull. And contacts to the Ascendant, the point that governs first impressions and the body, often explain that hard-to-name sense of recognition, or its absence, when two people first meet.
None of these is a verdict on its own. A single difficult contact does not doom anything. A page full of easy ones does not guarantee it lasts. The chart describes the raw material. What gets built from it is not in the chart.
Why "compatible" is the wrong word
Here is the part that matters most. It is also the part the compatibility apps get wrong.
The easy contacts in a synastry, the ones traditionally called harmonious, describe where two people flow without effort. That sounds like the goal until you notice that effortless can also mean unchallenging. Some of the most enduring relationships I have read for are full of friction between the charts, the kind of contacts that make two people rub against each other and grow. And some of the smoothest pairings on paper describe two people who like each other easily and never quite deepen.
A chart heavy with tension is not a doomed match. A chart full of ease is not a guaranteed one. Both are just descriptions of what the relationship will tend to feel like and ask of each person. Friction between two Mars placements might mean you argue. It might just as easily mean you energise each other. Which one it becomes depends on how the two people handle it. That is exactly the part astrology cannot decide for you.
This is why I am wary of any reading that scores a relationship out of ten. The number flatters the buyer and tells them almost nothing true.
One point of confusion worth clearing up. Synastry compares two separate charts and keeps them separate, yours and theirs side by side. There is a different technique, the composite chart, which merges the two into a single new chart representing the relationship itself as a third entity. They answer different questions. Synastry asks how these two people affect each other. The composite asks what the relationship is, as a thing in its own right. A full reading often uses both. They are not interchangeable though. A lot of online material blurs them together anyway.
What you can look at first
If you want to start with your own two charts, the first thing to find is each person's Sun, Moon and rising sign. Then notice where they fall relative to each other. Does one person's Moon sit in the same sign as the other's Sun? Is there a Venus close to a Mars? Those early observations will not give you a full reading. They will show you the kind of thing synastry pays attention to.
For that you need both birth charts drawn accurately, which means each person's date, time and place. You can build a full chart for any date with AstroVisor 3D, the free 3D viewer that lays out the planets, the houses and the aspects between them. If you do not have your own placements yet, the free Sun, Moon and Rising Sign Calculator gives you the three anchors in about a minute. And when you want the full picture, how two charts actually meet across every contact and house, that is what a synastry reading is for. It goes well past what any compatibility score can tell you.