*DO NOT GET HUNG UP ON PLANETARY ASPECTS.* They’re not the thing most indicative of the nature of the person. The late David Roell of Astroamerica.com (may he rest in peace) was a traditional astrologer I can immediately think of as being the one to drive this idea home the most. The importance of aspects between planets is not top priority. Planets, their placement in the houses, then relationship to each other, then placement in the signs, is the order of priority. Not all aspects are created equal either. The T-square is one, however, that does make it to the top of the list of planetary aspects that should receive attention, because the tension will alter the way that the planets function and how the native uses them.
The general consensus is that the T-square, composed of two planets in opposition that are both squaring another planet, is a planetary aspect pattern in which the two opposing planets allow the tension between them to funnel onto the planet squaring them. The planets are (usually) within the same mode. Thus, the solution is to “fill in” missing sign in the same mode. This is usually done for the native with a Grand Cross, who has two oppositions in his or her chart, with the planets in one opposition making squares to the planets in the other opposition. If you connect the planets with lines, a square will appear.
The T-square connected is a small triangle, half of a square. This aspect pattern is fairly common, and seen in the charts of many successful people, so it is written. Well, okay, that makes sense: people who have this sort of tension have a need to release the energy of the planet most under pressure, so they would be driven to succeed. However, I’m hesitant to say that the T-square is indicative of a person who will be successful. Driven, perhaps, to something, somewhere, that may or may not translate to worldly success. This drive may not manifest into something in the physical world that the rest of us can appreciate. To say that the T-square is correlated with success is sort of like making the mistake of saying that planets or angles connected to the Aries Points are indicative of fame. The T-square is an itch to scratch, and if we need a socially appropriate outlet, or if we happen to have been born in a station in the world that allows us an outlet that can bring home the bacon, then well, okay, success.

My definition of success has changed as I’ve grown older. What fortune and fame we may dream of as children, when we think we are boundless, or when dream in defiance of our elders or culture may not live on. Yes, there are folks who dream of being a rock star and work hard all of their lives to achieve their dreams, but even for these people, something must have changed along the way. Perhaps they fell in love and were inspired in songwriting. The music changed. Perhaps they were exposed to a different genre of music along the way. Perhaps the native DID achieve fame, and realized that (shocker!) there may not be as much money in it as you think. Really, now? But aren’t all famous people rich? If you are asking this question, let me be the first to introduce you to the concept of contracts. You may also be interested in learning about the concepts of promotion, swag, borrowed designer dresses and jewelry (do you honestly think that the 20-something who just made it big with her pop song actually *bought* that $750,000 Harry Winston necklace she’s wearing on the red carpet to wear once? You know who is successful? The people who don’t have to live life on someone else’s terms. This is success to me.
The T-square creates an itch to scratch. The opposing planets are in the same mode, so they hear each other, but they inhibit each other.  “On one hand….but on the other hand…” It creates an indecisiveness, or inhibits the normal action each planet takes such that the action may become maladaptive, depending on the nature of planets involved.. Saturn opposing the Moon inhibits the expression of nurturing, while the moon clouds the responsible, sober nature of Saturn with emotion. It may also, in time, make a person feel more responsible for their emotions, more stoic. On the other hand, Venus opposing the Moon may manifest as chasing relationships in which one isn’t emotionally fulfilled, or being such a social butterfly that you don’t make a lot of deep connections. The implications and the impact may not matter as much to someone who doesn’t want a lot of intimacy anyway. On the other hand, the amplified energy of the feminine planets makes for someone who may have exaggerated expression of each. When the opposing planets square another planet, you have not only the tension of the squares, but the tension of the opposition the planet.
Okay, this may seem really bad, but there is more than one way to look at something.

    Tension is not necessarily a bad thing, as being slack or at ease is not necessarily a good thing. For example, sexual tension. I’m loathe to think that most sexually mature adults don’t find something exciting in sexual tension. It may be inappropriate desire because of the parties involved, or because it cannot be acted on at the given time, or because of situational factors, like working in close proximity under high pressure. Sexual tension really only exists when there are two people feeling an unrealized sexual desire or fascination, however fleeting, however circumstantial. It may not be comfortable, but it is exciting, and it can be the foundation of a relationship. In the physical world, tension keeps bridges from collapsing. It keeps the roof over your head. It makes the knife cut, the fork stab, the teeth chew. There is a need forces to oppose in order to create change. A challenging aspect is challenging, but not detrimental if one doesn’t allow it to become that way.
So, the apex planet that receives the tension of the squares on either side of the opposition receives the force of not only the planets squaring it, but the energy of the tension between the planets. Some planets handle this better than others, as some materials handle a certain amount of opposing force more easily than others, in that a native may have the cultural assets and other native talents to handle the tension better. Someone with a flowing aspect to the Moon from Mercury may find that articulating her emotions helps her use the tension from her Moon/Venus square.
Since the planets in a T-square are (usually) in the same mode, they have the same energy expression. Cardinal planets want to start something new. Fixed planets want to keep things on an even keel. Mutable planets want to discuss and adjust. However, unlike with the Grand Cross, the T-square has a missing sign and house, as there are four signs in each mode. What is missing, what is opposing the apex planet, will be felt as a loss. Therefore, in order to ease the tension, in order to take action to acquire what is missing, the native may use the T-square like a witching rod, with the apex planet seeking out the hidden or missing thing in the opposite sign and house.
For example: A woman has a T-square involving and opposition between the Moon in Aries, Saturn in Libra, and both square Venus in Capricorn. First, take into consideration the nature of the planets, their houses and their signs. A T-square involving angular houses is more apt to show up in public life, career, and persona. They may share this tension with the world, and depending on where Venus is, push the energy toward that house. Let’s say that Venus is in the 10th house. Public life and public persona is where the tension goes. However, the 4th house, a point and degree in Cancer, is empty, on the opposite end.
This woman focuses her business saavy and need to be or find someone to be responsible (Saturn) infectious emotional energy (Aries) on her social charm and beauty (Venus). She can be debilitated by self-doubt and insecurity. However, she can draw in people who are willing to help her create emotional stability and sense of belonging (Cancer 4th house). She may literally find Cancerian types of people to fulfill this role, and may find herself, happily or not, with Cancerian types. How this sits with her overall depends on other factors in her chart, and what expression of Cancer that often comes out depends on what she uses her apex planet to divine. Of course, since filling in the Cancer point in the 4th house creates a new tension, it may not be what she hoped for. The people she brings in, for example, may be inhibiting. Maybe she gets a Cancerian boyfriend who doesn’t respond well to her nurturing and nurturing needs (Moon in Aries), is sick, weak, or miserly (Saturn in the 7th) and ultimately, inhibits her feminine expression (Venus), perhaps by trying to change the way she dresses or does her hair, or attempting to make her more demure or shy, than she naturally is, as Venus in the 10th house is rarely demure (unless it is a ruse), no matter what the sign.
The missing point in the T-square may be indicative of what was missing in life in the early years, especially if it is below the horizon. Cancer in the 4th house may be very well related to loss or denial of family, or family inhibition of her feminine expression. This may be especially so, given that Saturn and the Moon are involved. She learns to use her feminine charm to seek the love and sense of belonging and being at home that she grew up without.
Let’s try again. A man has Uranus in Scorpio in the 9th opposing the moon in Taurus in the 3rd. They are both squared to the apex planet, Venus, exactly on the cusp of the descendant. Ah, now the partner is involved. Whenever a planet is conjunct the descendant a partner is sought to fulfill this aspect of the native’s psyche. However, what can often happen, especially in youth, is that once in a partnership, like a committed relationship, the native engages in a sort of tug-of-war over the planetary expression. While another person was to represent that part of the native’s psyche, now, once the relationship has been established, seeks to take it from the other person. What he really and truly wants is to own that Venus himself, but for whatever reason, cannot. Perhaps his artistic side was never developed, or more pressing issues (like work, like health) came in the way. Either way, once he brings in the Venusian woman, he fights to become the Venusian. He suddenly needs full control over home decor, or insists she dresses or looks a certain way, or makes it difficult for her to create art, or thwarts her other relationships or competes with her for popularity. But why?
Perhaps if we look for clues in the opposition and what Uranus in the 9th opposing the Moon in the 3rd means, then we may get an idea. An opposition on the brainy, 3rd/9th house axis can indicate someone of some sort of brilliance, or at least the appearance of brilliance. This person was actually raised to think that he was a genius because of the erratic behavior of his mother and her inability to create a stable home life. As a child, this bright computer nerd ended up paying a lot of the family bills, and his parents, both without college educations, thought he was preternaturally brilliant. True to the nature of this opposition, he never went to college or appreciated formal schooling much, but he did become successful with computers and technology. He never really got a chance to explore anything else because he had a financial responsibility to his family.
Thus, he sought partners to become the undeveloped part of him that he wanted to embody, and once Venus was embodied, decided to embody it himself. His sense of who he was in the world was actually determined first by being a parentified child, then by being a breadwinner. Unfortunately, his Uranus could not relinquish control, and it wasn’t until he was in his mid-thirties that he realized that no, he wasn’t really a genius. He saw that people who took more traditional routes in his field were surpassing him because they had college degrees and had less fear of being without security (Moon opposite Uranus) in order to try new things. He was able to use his Venus to charm people into thinking that he was super intelligent (opposition to ascendant, Aquarius rising), and he could compare himself to his Venusian partner, who was painted as therefore not very intelligent. This was frustrating for both partners, and the relationship ended in divorce and alienation.
While the pattern of using the T-square as a witching rod to find the missing sign and house is typical, it can often manifest as dysfunctional. This is especially shown in both cases given, where the planet ruling the empty sign and house was involved in the T-square by opposition, in which both parties try to bring in people to represent what is missing. The solution then is to find a way to fulfill the role him- or herself. But how?

The ruling planet of the missing sign and house will give clues. If it is involved in the T-square, then it becomes more dire to express it. What is the nature of the ruling planet? If it is say, the Moon, see where the moon is by house, sign, dispositor, and condition. Is it in an expressive house, is it well-placed? Where is the dispositor, and what is going on with it? Is the planet intercepted, near an angle, conjunct sensitive points? For example, the woman with the Moon in Aries has the moon in the 1st house, but intercepted. The good news is that her lunar expression comes out when she deals with the public. The bad news is that it is not the first expression, but the one that comes after her Piscean expression no longer works. The bad news is that Mars is in Virgo. The good news is that it is conjunct both Jupiter and the North Node. The bad news AND good news is that it is also square the Sun. That is a lot of masculine energy and productive energy. She can create her own sense of home somewhere, whether it is literally, through architecture (Venus in Capricorn in the 10th, square Saturn in Virgo) or anywhere (Moon trine the Sun) and reinvent herself (Moon trine Uranus in the 8th).
As she gets older, as life keeps throwing more things in her way, she starts over and becomes a new person who creates a sense of home that she can take with her anywhere. She can nurture and protect herself, owning her empty 4th house. How did she do it? She took up genealogy, learned about her legacy, and decided that having roots, or a clan to belong to, was overrated, so she initiated (cardinal) a new sense of family, with her as the root.Not every T-square fits neatly into the modes. I haven’t read too many books that really delve into the issue of T-square in which there is a planet out-of-mode. This only happens when planets are at the very first few or very last degrees of a sign. For example, if one has a T-square in which Saturn is at 25 degrees Virgo, technically opposing the Moon at 1 degree Aries and both squaring Venus at 2 Capricorn, Saturn, being the mutable sign, is out of mode.  I think Stephen Arroyo‘s explanation is probably best. This doesn’t make the T-square easier to handle, but rather adds a new dimension to it. The mutable planet is adding the dimension of flexibility and adjustment. It creates a more nuanced T-square, not an easier, or null-in-void T-square. There is a planet involved that speaks a different language with the Saturn in Virgo, thus directing the witching rod in different directions.
Overall, the idea that energy just builds up and bursts only through the apex planet is only half of the story. To have a T-square is to feel incomplete, to have an itch that needs to be relieved. If we are to have a sense of harmony about ourselves and our lives, and to feel that there is a healthy equilibrium, we need to have a sense of balance. If there is so much energy focused on the apex planet, the drive is to balance it by turning energy toward the unknown wellspring that could be on the opposite end of the zodiac.