Square
A square is two bodies 90 degrees apart, in signs of the same modality but incompatible elements, and it is the aspect of friction: two planets that want the same kind of action for different reasons.
The shared modality is the key to reading it. Two cardinal planets in square both want to begin, and collide over what. Two fixed planets both refuse to move. Two mutable planets both keep adjusting, and neither settles.
Traditional astrology counts the square among the hard aspects and modern practice tends to add that it is the productive one, on the grounds that friction is what makes anything change. Both readings are in the sources; neither predicts an outcome.
Frequently asked
Are squares bad aspects?
They describe tension, not misfortune. The classical label is hard, and hard means demanding rather than damaging.
Why do squares repeat a theme?
Because the two planets share a modality, so they keep meeting the same way rather than resolving into different roles.