(This is a synastry lesson. If you prefer, see Synastry Interpretations instead.)
Fire and air are compatible with each other, and they are the yang elements. Earth and water are compatible with each other, and they are the yin elements.
In general, we can see eye to eye with people of our compatible elements. It’s best to start with compatible Sun-sign elements (both yin or both yang). This is really the ideal. Some astrologers say there are so-called compensations for incompatible Sun elements that can make for a bearable relationship. But we tend to disagree. Yang (fire & air) and Yin (earth & water) just don’t see eye to eye. This isn’t necessarily bad, but you will spend a great deal of time explaining yourself to each other. If you are dominant yang energy, you will notice that you need to put in extra effort to understand a yin person, and vice versa. Communication will not be smooth in such a relationship.
Being in a long-term relationship with the wrong energy can actually be detrimental to your health. It’s doable, but not always healthy.
The ideal is compatible Sun elements. If the Suns are compatible, everything else can be worked on. And this is not a asking too much — about half of the population is compatible with your Sun element. You can check which Sun signs are compatible with your own on the Zodiac Signs Compatibility page.
In addition, compatible Moon elements, Rising sign elements, and Venus sign elements are desirable. The more the better. Compatibility is all about this elemental energy flowing smoothly. Incompatible elements are like gears of a transmission grinding against each other instead of flowing smoothly. You can actually test this and get a feel for incompatible energies: take a 4-hour car drive with a person who has an incompatible Moon element.
The four elements express and understand love differently. Compare the love styles of the 4 elements be reading these articles:
Fire Sign Love Language
Water Sign Love Language
Air Sign Love Language
Earth Sign Love Language