How synastry works
Synastry starts with two complete birth charts. It then measures the angle between every planet in one chart and every planet in the other. When two planets form an aspect, like a conjunction or a trine, astrologers read it as a theme in the relationship. One person's Moon trine the other's Venus, for example, is traditionally read as easy affection and care.
Synastry orbs are usually tighter than in a single chart, because two charts produce many more possible contacts. This site uses 6° for conjunctions and oppositions, 5° for squares and trines, and 3° for sextiles.
Which connections matter most
Sun and Moon contacts describe how two people's core selves and emotional needs meet. Venus and Mars contacts are the classic signature of attraction. Mercury contacts describe everyday understanding. Saturn contacts describe commitment and weight. Contacts between slow outer planets, like Neptune and Pluto, are shared by almost everyone born in the same years, so they say little about a particular couple and are left out here.
House overlays
If both birth times are known, you can also see where one person's planets fall in the other's houses. A partner's Venus in your seventh house, for instance, is read as bringing affection to how you approach partnership. Without both times, overlays are left out rather than guessed.
Why there's no compatibility score
Many sites turn synastry into a percentage. We don't, because the number hides more than it shows. Friction between charts isn't a bad sign, and plenty of lasting relationships have more squares than trines. Synastry is better at describing what two people bring out in each other than at grading whether they should be together. Whether a relationship works depends on choices, not charts.