Moon Sign compatibility in astrology is one of the most crucial factors between two people. It can make or break the relationship unless there are strong mitigating factors.

The Moon rules over Cancer, the sign of emotions, and if the two Moon signs are incompatible, there will be an emotional discomfort which can wreck the relationship, especially if the two live or cohabit under the same roof for any long period of time.

Depending on the aspect, two incompatible Moons will have different ways of expressing their feelings as well as different security needs. If the two Mercurys are compatible they may be able to talk it out and come to some kind of understanding but the tension will still be there.

One's Moon conjunct, trine, sextile, or oppose (by sign) the other Moon will establish a functional emotional rapport whereas the square, semi-sextile, or quincunx (by sign) will not. The conjunction is the easiest and the square the most difficult. Simply said, fire and air Moons all get along but not with earth or water.

In terms of placement quality, the Moon works best in earth or water with Taurus and Cancer leading the pack. These two signs of exaltation and dignity give the best emotional stability or sensitivity. Fire Moons tend to be overly impulsive and aggressive whereas air Moons tend to be too superficial, changeable or insensitive.

I have also noticed that the most successful politicians, business people, or professionals in general have the Moon in an earth or water sign. The Moon here gives greater depth, intelligence, sensitivity, and tenacity.

If your two Moon signs are not compatible do not lose hope, especially if it is between a couple or love partnership. A good Sun/Moon link can do much to save the relationship, especially the conjunction or trine. Moon/Venus links are also helpful. In short, the entire chart(s) should be looked at for possible ways out.

There are certain exceptions in astrology when it comes to compatibility, whether it's between Moon signs, Sun signs, or any other combination. The exceptions are between signs belonging to the same planetary ruler.

Moon in Taurus, for example, may get along with Moon in Libra because both these signs are traditionally ruled by Venus. The same can be said between Aries and Scorpio which are both traditionally ruled by Mars, and Capricorn and Aquarius which are traditionally ruled by Saturn.

The signs ruled by Mercury (Gemini and Virgo) and by Jupiter (Sagittarius and Pisces) are more difficult because of the square relationship. A final exception is Leo and Pisces which often works for some reason.

While the above exceptions are not as compatible as those by element, they must be pointed out, as long observation bears them out.