Square

This is a 90 degrees aspect. The planets involved in a square are located 90 degrees apart from each other in the Zodiac.
This is a tough combination, because the two planets involved do not understand each other easily. You could say they don't see each other, because they are standing 'around the corner' from each other.
It therefore takes a lot of effort to get the energies of the planets to cooperate and blend well.