The square or 90 degree aspect is often thought of as a tension aspect between two areas in life that do not experience harmony with each other. To understand this aspect thoroughly, we should first understand that it is almost always between two signs of the same mode.
Thus a planet in Aries will, most of the time, find its square aspect, to a planet in Cancer or Capricorn,- both of the Cardinal mode,- the same as Aries. If a Taurus planet is experiencing a square it will most likely be in either Leo or Aquarius, both of the Fixed mode,- the same as Taurus.
One might think that if two planets are in the same mode they would experience a natural agreement and not the kind of friction and tension that a square seems to represent. It is not so much that the same mode is disharmonious, for in fact, it is not. Where the disagreement occurs is that the two signs in square aspect to each other, although they may want the same things, disagree terribly about how to get those things.
As a result, if an individual wants achievement he would if he were an Aries simply charge ahead through life, with little regard for the consequences (because of the Mars rulership of the sign) whereas a Cancerian (also a cardinal mode sign) would try to achieve it by making others into their extended family and then emotionally try to enlist their help. At the same time, a Capricorn (also of the cardinal mode) would try not to act emotionally at all, or in any immature fashion that might detract from their desires, and instead apply themselves through extremely long and tediously hard work.
At any point through the zodiac, the square aspect represents at odds methods of achieving the same thing. What is even more interesting is that these 90 degree distances between signs also symbolize houses with the same distances. To understand the difference in outlook between a Libran, for example and a Capricorn ( two signs in 90 degree square to each other), we would also have to understand the natural houses involved. Libra rules the natural Seventh House of marriage and partnership, while Capricorn represents the Tenth House of fatherhood, career and standing in one's community. As a result, where Libra would try to do things in partnership to achieve goals, Capricorn would rather give orders to inferiors or wind up doing things themselves.
Thus, we can see that some of the differences between planets in square to each other, regardless of where they occur in the horoscope, are not so much a disagreement insofar as what the individual wants out of life, but rather what means they are willing to use to achieve those goals.
Planets involved in a square will act differently, depending upon what signs and houses the square falls in. The resultant course of action from squares to Libra will be quite different than squares to Aries, Cancer or Capricorn.
Think of the other Modes and the same thing occurs. In the Fixed Mode, represented by Taurus, Leo, Scorpio and Aquarius, these signs have very different approaches to life, even though their end goals might be similar. Planets in Leo receiving squares would love a good fight, especially if others know about it and can be the audience, while similar planets in Aquarius receiving a square, would deal with the same tensions by walking away. Taurus and Scorpio would never deal with tensions in the same way. Taurus would rebel immediately where Scorpio could seethe for years at a time without saying a word until the time is right.
In the Mutable Mode, represented by Gemini, Virgo, Sagittarius and Pisces, the tensions of squares bring about changes on different levels. Gemini changes its ideas and thought patterns. A virgo receiving a square would change their methods and how they function. Sagittarians might change their location and Pisces would change their beliefs and dreams. Again, it is not that these individuals are so different, for in fact they are not insofar as what they want out of life. It is mostly how they think they can go about getting it.
