The word synastry comes from the Greek prefix "syn" - bringing together and "astron" - a star. It is the comparison of two or more horoscopes to determine compatibility. Any relationship can be analyzed synastrically (business partners, parent-child, teacher-student, etc.), but here remarks will be specific to love/marriage relationships. (Not surprising, since at this moment, Mercury and Venus are conjunct in Libra in my 5th house, trine my ascendant and natal Mercury in the 8th.)
The prime significators of the wife in a man's chart are Venus and the Moon. In a woman's chart, the prime significators of the husband are the Sun and Mars. The sign and house placement of these planets in the nativity, as well as placements of and aspects to the rulers of the 5th and 7th houses, indicate the type of mate the native is likely to attract or be attracted to. Consequently, cross aspects (aspects between charts) to these planets are frequently seen in love relationships. Psychologists and psychiatrists since Freud have observed that people often choose mates who are very similar to their parent of the opposite sex. This is easily understood considering that the Sun and Moon are also prime significators of the father and mother. The parent/child relationship may be played out repeatedly in later adult relationships until the individual eventually resolves his/her family-of-origin issues. Astrological counseling can be valuable in this regard. With the help of the parents' and siblings' natal charts, client and astrologer can examine the early family dynamics and possibly discover outmoded behavior patterns that have hindered the individual's evolution.
The human need for relationship on an emotional and spiritual level is in part a search for wholeness. We seek from others that which we feel we lack. The 7th house shows areas where we may deny our own personal characteristics and project them onto others, particularly our mates. The danger in this is that we may eventually resent the person for the very same reasons we chose him/her in the first place! Obviously, until we acknowledge and "own" all that we are, it is difficult to establish harmony in relationships with others. Sometimes this feeling of "lack" has a genuine astrological basis. People with no planets in water signs, for instance, may be attracted to people with many water planets. (Of course, no chart is ever truly devoid of any element. A chart with no planets in water signs will undoubtedly have planets in "water houses".) I have been fortunate in my practice to have had the opportunity to work with "astrological twins". Their charts have no planets in earth signs and a cardinal t-square involving 6 planets. One of them is married to a woman with Moon/Jupiter/Ascendant in Capricorn; the other was involved with a woman with Sun/Jupiter/Venus in Capricorn. Their strong t-squares attracted the missing element of a grand cross. Talk about wholeness!
I've also seen instances where a person with a very strong opposition (two or more planets conjunct, in opposition to another conjunction) falls in love with a person who has a major planet or planets t-squaring that opposition. Or a person with a strong square attracts someone with planets in opposition and square that configuration, again creating a t-square by cross-aspect. A multiple planet trine may attract the third element to complete a grand trine; however, despite popular "sun sign" books which advise, "If you are a Leo, your most compatible signs are Aries and Sagittarius", I've noticed sun trines occurring more often in platonic friendships than in love/marriage relationships. A certain amount of friction is desirable (indeed, necessary on the physical level!) in sexual relationships. In my observation, the most powerful cross-aspects in love relationships are the man's Moon or Venus conjunct or oppose the woman's Sun, Mars, ascendant or ruler of the ascendant; and correspondingly, the woman's Sun or Mars oppose or conjunct the man's Moon, Venus, ascendant or ruler of the ascendant.
There are apt to be many cross-aspects in the charts of lovers or marriage partners. Bear in mind that the "hard" aspects (squares and oppositions) do not always forebode great difficulties, nor are the "soft" aspects (sextiles and trines) always favorable. For instance, if a man's Saturn t-squares his partner's Moon/Uranus opposition, he may provide a grounding, stabilizing factor to her emotional fluctuations; whereas, a man's Jupiter trine his wife's Venus may cause him to be overindulgent with her, lavishing her with expensive gifts and spending far beyond his means. Of course, these examples will not necessarily manifest thusly - ALL factors between charts need to be taken into consideration.
Synastrically, planets in mutual reception (each planet in the sign of the other's ruler) have a certain sympatico. Therefore, one partner's Saturn in Aries and the other's Mars in Capricorn, even though they may be square by orb, could work well together - the mutual receptivity mitigates the tension of the square. An affinity also exists between planets in antiscion degree (i.e. at the solstice point - equidistant from the Capricorn/Cancer axis). While an Aries Sun and a Virgo Moon may not be thought of as particularly compatible, if the Sun is at 27 degrees Aries and the Moon at 3 degrees Virgo there is a definite link. Midpoints become important in cross-chart analysis; for if a planet in one chart conjuncts the midpoint of two planets in another chart, those two planets become "aspected" WITHIN THE CONTEXT OF THE RELATIONSHIP, even though no aspect exists between them in the individual chart.
When the planets of one nativity are placed in the appropriate houses of the partner's chart, and vice versa, we get a more detailed picture of the areas of life where these energies will have their greatest impact; once again the 5th and 7th houses are critical, as well as the 4th, if the partners live together. Be aware that a planet in any sign will have a strong character resemblance to a planet in that sign's natural house. Thus, Mars in Aquarius in one partner's chart will function similarly to an 11th house Mars in the mate's chart, regardless of its sign placement.
Perhaps the most important point to remember in synastric analysis is that personal growth and evolution are the reason we're all here in this schoolroom called planet Earth. Most of our lessons are learned in the context of our dealings with others. The strength and longevity of any given relationship depends to a large extent on the level of maturity and degree of personal commitment of the individuals involved.
Bio: Jacquie Smith is a writer and astrologer specializing in synastric relationship counseling. She is currently writing a book entitled "The Neptune Factor: The Astrology of Addiction and Recovery."