What synastry compares
Synastry is the branch of astrology that sets two birth charts next to each other. Instead of reading one person, it looks at how one person's planets line up with the other's: whose Venus touches whose Mars, whose Moon sits near whose Sun.
The comparison can be made for any pair: partners, friends, siblings, coworkers or a parent and an adult child. We offer it for fun and reflection, as a way to talk about differences and shared ground, not as a score for a relationship.
Inter-aspects, the heart of the tool
An inter-aspect is an angle between a planet in one chart and a planet in the other. We check the major ones: conjunction (0 degrees), sextile (60), square (90), trine (120) and opposition (180), each within a set orb.
Astrologers read conjunctions as blending, trines and sextiles as easy flow, and squares and oppositions as friction or contrast. Friction is not read as failure. Many astrologers say a few challenging links keep a bond lively and help two people grow.
Contacts astrologers often check first
Sun and Moon links are traditionally read as a sense of basic understanding. Venus and Mars contacts are associated with attraction and style of affection. Mercury links describe how easily two people talk, and Saturn contacts are read as commitment and also as weight or duty.
When birth times are known, the tool also shows which house of one chart each of the other person's planets falls into. That overlay is read as the area of life where one person tends to affect the other.
Why we give no verdict
Plenty of compatibility tools hand out a percentage. We do not, because astrology has no tested way to measure whether a relationship works. Real relationships depend on choices, values and communication that no chart can capture.
What we offer instead is the full list of contacts with plain notes on how the tradition reads each one. Use it as a conversation starter, not a decision.
Related comparisons
Synastry keeps two charts separate. A composite chart blends them into one chart for the relationship itself. For a quick look at each person first, the Big Three calculator shows Sun, Moon and rising, and the Venus sign calculator covers style in affection.