As astrologer Annie Heese writes, synastry is a pillar of "relationship astrology," or the practice of analyzing couples' birth charts to determine the course of their relationship. Rather than look at each chart separately, astrologers will generate a new chart that incorporates both partners' astrological info (where the planets appear in the signs and houses at the moment of each person's birth) all on the same wheel. With couples' respective charts displayed this way, astrologers can look for similarities, differences, and complementary features more easily.
All of the planets are analyzed during a synastry reading, but Venus and Mars, the respective rulers of love and sexuality, carry a little more weight. Your astrologer will check to see where your Venus and Mars are in relation to your partner's — are they parallel to each other, do they overlap, are they across the wheel from each other?
Without getting too geometrical (some astrologers base their conclusions on the angles that your planets make with your partner's), there are a few planetary connections that are believed to be more meaningful than others. Spoiler alert: Charts that align perfectly don't necessarily spell everlasting love.
If your Venus is where your partner's Mars is located (or vice versa), it means your attraction is mutual — and your flirting style probably complements what they find sexy. Meanwhile, if your partner has a planet in a house that's empty in your chart, they enhance an area of your life that was pretty hum-drum before you met.
Finally, one of the most powerful (and visible) connections that can occur between your partner's chart and your own is when you share a moon sign. This connection indicates a deep, emotional understanding between you and your S.O. You probably process feelings very similarly and know how to help each other through difficult times. And on a lighter note, it's likely you're the same kind of strange when it's just the two of you.
As is the case with astrological topics, whole books could be written about synastry — we're just scratching the surface here. If you want a better idea of how this practice works, we recommend seeing it in action. Draw up your own synastry chart here — you'll need your birthdate, time, and place, plus your partner's — and discover what the planets have in store.
Of course, if you and your S.O. seek out your synastry chart only to find the stars spell nothing but gloom and doom for your future, don't stress. It won't serve you to spend your everyday life worrying over the heavenly bodies. And anyway, there's probably another compatibility theory out there that swears you two are meant to be.