Like most (or all) astrologers, I grew up on the modern notions of planetary rulerships - ruler, detriment, exaltation, and fall - where Uranus, Neptune, and Pluto have rulership roles. Somewhere along the line, I rejected the transpersonal rulerships - because using them violated the symmetry of the rulership model. Later as I began looking at those three planets in a different way, their rulership of anything didn't really make any sense (Chiron and the asteroids also fall into this category of bodies that don't rule anything).
As an aside, the "rulership" of Aquarius by Uranus is what led to the notion of Aquarius being "revolutionary" - a notion that is not true but one that is treasured by many - particularly those of us in the Pluto in Leo generation (some still cling to the fantasy that we are revolutionaries). This "notion" is the genesis of the name of this blog. "Aquarius the Revolutionary" is the first "Cherished Pet"
Again, like many astrologers, I've been introduced to both Medieval and Hellenistic astrologies in the past 10 years or so and "rulerships" have been replaced with the broader concept of planetary dignities. This concept expands beyond ruler/detriment, exaltation/fall and presents us with confines/terms, triplicites, and face (decans). So much dignity! How do we sort through all this to come up with something that makes sense to us.
I think most astrologers studying Medieval or Hellenistic have an initial impulse to fully embrace all the teachings and tenets of those lineages; but at the end of it, we are modern people with modern notions and both those older lineages present ideas that challenge our current practice and thinking. Dignity is one of those notions. The other is the idea that the native is limited by their chart - "NOT all things are possible". That borders on heresy in this modern Western culture.
The latter idea is the more important one here, but planetary dignity is one of the tools necessary to determining those limitations.
Modern astrologers view rulership/exaltation/detriment/fall in a way that seems different from how they are viewed by the older schools. Fundamentally, we view rulerships and exaltations as good and detriments/falls as bad. I've come to the view (which is essentially Hellenistic) that good is better expressed as comfortable/effective and bad is better expressed as uncomfortable/ineffective. This is a subtle but significant change. It allows a planet to be in a position of strength or weakness and still makes it possible for that planet to do good or ill by its own nature.
Capricorn, for example, is a powerful place for Mars (exaltation) but it is no guarantee that it will behave or do well. For every Dwight Eisenhower who could plan and execute a D-Day invasion, you have a George Custer who obviously couldn't.
One question I still wrestle with is the difference between rulership and exaltation. I know the explanations that the ruler owns the resources of the sign and that the exaltation is like an honored guest but I always need something a bit more easily understood. So far, the best expression of the difference has been that the ruler has responsibility for the affairs of a sign and has the means to handle that responsibility. Whereas, the exaltation is the actual owner of the sign and enjoys its resources without the burden of responsibility. That, in a way, gives it more freedom of expression.
I lean toward the latter explanation. It seems to me that the sign a planet rules is a sign that allows the planet full expression of its nature, whereas the exaltation is that sign where a planet can use that nature to its fullest effectiveness.
I don't believe that planets are weak in their detriments and falls (very possibly the opposite) but that those positions create levels of discomfort because they represent impulses opposite to those of the nature of the planet. Jupiter is happiest in Sagittarius where it has free reign to express both its imagination and expansiveness - yet it's not so happy in Gemini where the imagination and expansiveness also have free reign. In Sagittarius the focused nature of the sign allows for better and more satisfying results. It is more possible to concentrate on an imagined project and bring it to conclusion. Whereas, in Gemini, the energy diffuses into nothing too rapidly. The imagined project can be started and sometimes even completed - but Jupiter is too easily distracted. The tendency is to lose interest and patience and move on too soon to the next great project. Subsequently, less is accomplished, dreams less often realized, and the levels of frustration and disappointment more readily elevate.
One also needs to consider that except for the Sun and Moon, each planet rules two signs - each sign signifying the opposite side of the same impulse - sort of an active/passive, masculine/feminine expression of those impulses. Mars rules Aries and Scorpio. One is aggressive, direct, and in your face. The other is aggressive, indirect, and behind your back.
Venus rules Taurus and Libra. One is languid, physical, pleasure/comfort oriented. The other is refined, mental, and oriented toward balance and symmetry.
There are still confines (terms), triplicities, and face (decans) to consider. Now I've found no space in my life yet for triplicities or face (except for occasionally losing it), but I have found that confines are a very powerful consideration in determining the function and capacity of anything in the chart that has a sign degree attached to it.
A confine (term, bound) is a planet that imposes a restriction or limitation on the expression of a planet. A Moon exalted in Taurus bound by Venus is going to be different from a Moon exalted in Taurus bound by Saturn. Both have the magnetic attraction of the Moon in Taurus - but the Venus bound Moon exudes a warmer, more feminine energy (Roslyn Carter, for example) than the same Moon bound by Saturn (Katherine Hepburn).
The clearest example I can give of how bounds work:
I own my own house and do what I want in my house. I am the ruler of my house.
My wife "owns" our house and has veto power over any decision she is genuinely against. She is the exaltation of our house.
We live in a university neighborhood surrounded by students. Students party whenever they can get away with it. Student parties start at nine in the evening and require a minimum of eight to eighty people. They play beer pong, drink flavored vodka, and don't smoke nearly enough pot. They get loud and obnoxious. The official party music requires a base line that travels as a wave through the ground and up your spine. Conversation at these parties devolves into guys bellowing and hooting and girls screeching and shrieking.
The students are our bound lords. We have no choice but to deal with them. I rule my house but I still have close windows when I don't want to, engage in conversations about civic responsibility at 2 am, and occasionally call the police (which I don't enjoy). Our bound lords affect our ability to fully express ourselves in our own domain. No choice.
Of all the dignities, I find the bounds (confines/terms) the most useful in terms of refining how I look at planets, and points. If you don't use them, I strongly encourage you to start experimenting with them (I use the Egyptian rather than Ptolomy's).
One final word around dignities (and Demetra George pointed this out to me). Each sign has a ruler but not each sign has an exaltation. That means that there are signs that a ruling planet does not have to "share" with an exaltation. These are the signs where a planet is happiest: Sun in Leo, Mars in Scorpio, Jupiter in Sagittarius, Saturn in Aquarius.
Taking Demetra's idea further: Mars, Jupiter, and Saturn will have positions in those signs where they are the bound lord (the Sun is always bound). These positions are:
Given that these three planets in those postions have to account for no exaltation lord and are bound only to themselves, this suggests that those positions are the purest expression of those planetary energies - for good or naught. I have not explored this deeply yet (having just recently come to it) but it is an interesting idea to continue exploring.