Trines
Western astrology assumes that each sign of the same triplicity is 120 degrees apart, forming angles to one another called trines, which are each equivalent to the 360 degrees of the circle divided by three. Trines are considered to be very powerful and yet very comfortable, free-flowing connections. This suggests that the signs in each element are very comfortable and compatible with one another and tend to have many of the same qualities in common.
Read more about this topic: Triplicity