I’ve spent the last 10+ years specifically studying relationship charts, and this is what I’ve noticed over and over in marriages and other long-term relationships, drawing on at least 500 – 600 charts I’ve looked at (if not more than that… I’ve lost count):
-Mars square (or sometimes conjunct) Saturn. This pops up SO MUCH it’s mind-boggling — at least 85% of the married synastry charts I’ve looked at have it. I think it attests to the drive (Mars) for responsibility (Saturn) set in motion by the friction of the square. It’s not often a pleasant aspect but it works like glue.
-When there’s no hard Mars/Saturn aspect, there’s almost always a conjunction between Saturn and the sun or Saturn and the ascendant.
-Alignment of the angles. The most common is one person’s AC conjunct the other’s MC or IC. Less common but still occurring is AC conjunct AC and AC conjunct DC. I’ve noticed that with marriages, it’s much more likely to be AC conjunct MC, whereas shorter-term relationships, or relationships people have when they’re younger, involve AC conjunct AC or DC. I think this is because the MC/IC axis involves more than just attraction and represents inner/domestic life as well as public life, which tend to become more important in marriages.
-There is usually at least one significant nodal conjunction (either with a planet or angle), but no consistency as to what that conjunction is. Node conjunct Saturn seems more common than node conjunct a personal planet, and nodes conjunct the angles (either AC/DC or IC/MC) also occur with some frequency.
-There is almost ALWAYS at least one planet/angle conjunction, often more. Conjunctions to the IC and MC are just as common, if not more so, than conjunctions to the AC and DC.
-I have not noticed any of the classically “good” aspects like sun conjunct moon or Venus conjunct Mars appear in marriage synastry any more often than they would by chance. Sextiles between Venus and Mars are actually the most common aspect I’ve seen between those two planets, but often there is no aspect at all.
-I have seen many marriage synastries with no major sun/sun, sun/moon, or moon/moon aspects at all, and in these cases there’s often a lot of Saturn action going on.
-There is often a major, tight conjunction between a personal planet or angle and one of the following asteroids: Union, Amor, Valentine, Eros, or Psyche (or these asteroids will aspect each other). Eros/Psyche conjunctions aren’t particularly common in marriages, but seem much more common in relationships that don’t culminate in marriage.
-Likewise, in the charts I’ve checked with name asteroids, there is often a significant conjunction of one person’s name asteroid with the other person’s name asteroid or personal planet, OR one (or both) people have the other person’s name asteroid conjunct a personal planet in their natal chart, especially the sun. Sort of like finding this person was “written” into their chart from birth…
-Chiron trine, sextile, or conjunct Venus is REALLY common. Chiron trine, sextile, or sun also occurs with some frequency.
-Tight orbs don’t seem to matter. When the relationships have sun/moon or moon/moon aspects, for instance, orbs of up to even 8 or 9 degrees seem to function just fine for compatibility. There usually are only a few aspects that are close to exact, and those aspects really set the tone of the marriage.
I have a theory that lots of tight aspects and exciting conjunctions eventually become too intense and uncomfortable, unless both people are VERY mature, and either cause two people to start repulsing each other or force the relationship to break apart for other reasons. This has been my experience time and time again. For a relationship to progress into a marriage, there needs to be some Saturn action, growth-oriented challenging aspects, a few nodal or angle connections, and significant progressions or transits to kick things off.