Synastry is the practice of comparing the birth charts of two people for purpose of determining the nature and sometimes the future course of their relationship.

The word is derived from the Greek sunastria (συναστρία), which means something like “co-starred” or perhaps “star-crossed”.

Synastry is usually used for the purposes of determining the compatibility between two people who are in a romantic relationship, although it is often applied more generally in order to study the strengths and weaknesses of any generic type of relationship between two people.

Relationship comparisons through synastry are usually accomplished by superimposing one person’s birth chart over another, either so that the planets of each are in the same wheel, or so there is one wheel just outside the other, allowing for quick comparison of where the planets in one person’s chart are relative to the the planets in the other person’s chart.