The square is an aspect of ninety degrees, formed when two planets stand a quarter of the circle apart. It is one of the five major, or Ptolemaic, aspects, and the foremost of the hard or challenging aspects. Ninety degrees links signs of the same mode but incompatible element, fire with water or air with earth, so the planets involved work at cross purposes, and the tradition reads the square as friction, obstacle, and the pressure that forces action. Yet it is not read as purely negative: the tension of a square is held to be the spur to effort and achievement, the resistance against which strength is built. It is given one of the wider orbs among the aspects. These are the interpretive conventions of the astrological tradition for this angular relation, in which the square supplies the dynamic challenge that the harmonious aspects lack.
Ptolemy's Tetrabiblos, compiled in Alexandria in the second century CE, calls the square a quartile, the figure of signs three apart, and ranks it with the opposition among the two hard aspects of the four he names in Book I.13, alongside the trine and sextile; the conjunction he treats separately as no aspect at all. Hellenistic practitioners such as Vettius Valens, writing his Anthology around 175 CE, worked the square by whole sign and read the relation as directional: of two planets so placed, the one standing in the tenth sign counted from the other was held to overcome it, an asymmetry that degree-based orb systems, which treat the square as a single mutual figure, generally no longer preserve. The wider orb usually granted the square reflects how forcefully the tradition has always rated this contact.