Interesting read..
KARMIC relationship clues:
One of the classic marks of a strong relationship is a close conjunction between one person's Sun and the other person's Moon. Squares and oppositions are also strong, but the soft angles of trines and sextiles are not strong indicators in and of themselves. However, if there are other indicators, then the trines and sextiles certainly add to the strength of the relationship.
Close contacts between one person's Sun and another person's Ascendant are also strong indicators of Soulmate possibilities. Conjunctions are strongest with squares and oppositions also strong.
Contacts between one person's Mars and another person's Venus are also very important in romantic and/or sexual relationships. (These would not be as significant in non-romantic or non-sexual soulmates, often called soul brothers or soul sisters.)
Sun/Mars and Sun/Venus contacts, and Venus/Venus contacts and Moon/Moon contacts and Sun/Sun contacts can also be important, although possibly not as strong as the previously mentioned contacts. Again, the hard aspects of conjunction, square, and opposition seem to carry more weight than trines and sextiles.
Sometimes Sun/Neptune contacts are indicators of soulmate relationships although many astrologers don't seem to say much about this one when discussing soulmates. At the very least, Sun/Neptune contacts indicate a feeling of merging into each other, which can be quite exhilarating. Often it is hard to tell where one person ends and the other person begins. The two people frequently have a strong psychic connection as well as a other types of connections.
Moving beyond planets and the Ascendant, there are some additional astrological clues for possible soulmate relationships. First, if one person's nodes are prominent in another person's chart, this is significant. Examples would be if the nodes of one person were square the nodes of the other person, if the nodes of one person were along the other person's Ascendant/Descendant, if the nodes of one person were conjunct/opposite/square the Sun or Moon of the other person.
Secondly, the Vertex is often used with synastry (chart comparison). If one person's Vertex makes a hard aspect to a planet or angle of the other person, this is a strong indicator of a karmic relationship, although not necessarily a soulmate one.
Thirdly, if the two persons have their angles square one another, this is a strong point of synastry. For instance, if one person's Ascendant/Descant is square the other person's Ascendant/Descendant, or if one person's Ascendant/Descendant is conjunct the other person's Midheaven/IC, then this is a strong karmic indicator, usually a Soulmate one of some kind.
Another indicator, one not commonly discussed, would be a similarity of pattern. In astrology, we talk about patterns that the planets make in a chart. Such patterns are called bowls, buckets, kites, yods, and so forth. If two persons have similar patterns and if those patterns have similar distinguishing marks, that could be a good indication of soulmate probability. For instance, for one soulmate couple that I know, both persons had a kite in their charts. Kites consist of a Grand Trine (three trines), and then an opposition to one corner of the trine which sextiles to the other two corners. The end result is a pattern that looks like a kite. Kites are not very common, and for two people to both have kites in a relationship made me sit up a bit straighter. Then I kept looking. Both of the Grand Trines were in Air signs, although the planets were not the same nor were the houses. Finally, both Kites had the same planet as the opposing planet. Thus, not only did the two people have the same pattern, but both patterns shared two strong distinguishing features.
Yet one more indicator might be that the Part of Marriage for one person falls upon a significant placement of another person's chart. The Part of Marriage is calculated by adding the Ascendant to the Descendant and then subtracting Venus. Some astrologers also use the Parts of Fascination and Karma when looking for Soulmate connections. The Part of Fascination is calculated by adding the Ascendant and Venus and then subtracting Uranus. The Part of Karma is calculated by Adding the Ascendant to Saturn and subtracting the Sun. (It helps to convert the signs to 360 degree equivalents when doing this sort of work or having your computer program calculate these parts for you.) I'm writing all of this off the top of my head, not from prepared notes or a text, so I'm sure that I'm leaving out some significant soulmate indicators. However, this is enough for even an astrological novice to see that there is no one definitive marker of soulmates in astrological charts. If you and another person share several of these features between your charts, chances are that you are soulmates. However, remember that not even soulmate relationships exist happily ever after without a great deal of work and effort. Good luck, best wishes, and happy hunting.
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That last one is interesting, the "shape" of our charts.