If you have been reading about relationship astrology for more than about ten minutes, you have run into both words. They get used as though they were alternatives, or worse, as though they were the same thing. They are neither. They answer two different questions, and only one of them is built on something you can verify.
Synastry compares two charts
Synastry takes your birth chart and your partner's birth chart and measures the angles between them. Your Venus against their Mars. Your Moon against their Sun. Every one of those angles is a real measurement between two real positions in the sky, and each one says something specific about how those two parts of two people meet.
Nothing is invented. Both charts existed before the relationship did, and they will still be true if the relationship ends. Synastry is the comparison, not a new object.
That is why a synastry reading can tell you where you connect and where you grind. It has two things to hold up against each other.
A composite chart builds a third chart
A composite works differently. It takes the midpoint between each pair of placements and assembles those midpoints into a single new chart. Your Sun at 12 degrees Cancer and their Sun at 12 degrees Capricorn produce a composite Sun at the midpoint between them. Do that for every planet and you have a chart that belongs to neither of you.
The idea behind it is appealing, and it is why the technique has lasted. A relationship does feel like a third thing in the room, with its own temperament, its own habits, its own moods. The composite is an attempt to draw that third thing.
The problem with the third chart
Here is what is rarely said plainly: a composite chart is not a picture of the sky at any moment. It never happened. The planets were never in those positions on that day or any other. It is an average, and averages of real things are not themselves real things.
The trouble compounds when you get to houses. Houses depend on an exact time and an exact place, which is how the sky was rotating overhead at one specific minute. A relationship has no birth minute. Astrologers who use composites disagree with each other about how to derive the houses, and there are competing methods that produce genuinely different charts from the same two people. When practitioners cannot agree on the method, that is worth knowing before you hang the result on a wall.
None of this makes composites worthless. Plenty of thoughtful astrologers use them and find them useful, and if a composite reading has ever landed for you, that experience is yours and no argument here takes it away. But it is a layer of interpretation built on top of an averaged chart, and it is a longer way from the ephemeris than synastry is.
Which one answers your question
It comes down to what you actually want to know.
How do we work together? Where does it flow, where does it catch, why does the same argument keep coming back? That is synastry. It compares two specific people and points at the contacts responsible.
What is this relationship like as an entity? If you think of the partnership as a thing with its own character, that is what a composite is reaching for.
The first question is the one people actually ask. Nobody wonders what their relationship would be like if it were a person. They wonder why he goes quiet when she wants to talk it through.
What we make, and why
Our Couples Compatibility Portrait is synastry, and the reason is the same reason we leave out a Moon when a birth time is unknown. Every placement on that portrait is printed by sign and degree, and every one of them can be checked against any ephemeris in the world. Run both sets of birth details through any free chart calculator and the numbers will match ours.
A composite portrait would look just as good on a wall and we could not offer you the same guarantee. We would be printing an averaged chart, using a house system that other astrologers would dispute, and asking you to take the result on trust. That is the opposite of how we want to work.
So the scorecard reads six real placements against each other, the five written sections are built on the aspects between two real charts, and if you want the numbers underneath, they are in the Score Report.
Try it before you decide
You can see what synastry actually reads like without paying anything. Our free chemistry check takes two birth dates, reads both charts against each other, and names what is between you. No account, nothing emailed to you, about a minute.
If it sounds like it knows you, the Couples Compatibility Portrait is that at ten times the depth: both chart wheels side by side, a scorecard across all six placements, five written sections and a closing verdict. Printed and framed, or as a print-ready file.
And the Couples Score Report is the ten dollar add-on that shows the raw numbers behind the verdicts, and what each one means for your two specific charts.