Obviously, it is the nature of the planet itself -and the sign or element it is in, plus a certain amount of free will or maturity on the part of the native(s) involved.

Fire and air planets will naturally gravitate towards one another as will earth and water. But what if that fire or air planet is in an earth or water sign?

A good knowledge of planetary rulerships(dignities) and sign/planet harmony(synastry) is needed to tackle this subject.

Basically, masculine planets and signs(Fire and Air) get along well with one another as do feminine(Earth or Water) planets and signs. This differentiation between masculine and feminine(or positive and negative/extrovert and introvert/etc.) is the most BASIC and IMPORTANT divisor and indicator of compatiblity or affinity in astrology. Everything else is an extensiion of it. Next in importance are the elements, modes, and polarities, and the individual signs or planets themselves. The chart below illustrates this.

Mars or Aries, for example, gets along best with itself, followed by the other two fire principles, and lastly the Air element. Of the air element, Earth/Libra is the most intimate because of the similarity of mode(both are cardinal). Fire does not get along with either the Earth element(which it finds too strong) or the water element(which it finds too weak).

Air is equal or complementary to fire but is superior or dominating to Earth and inferior or subordinate to Water. Earth and Water are equal or complementary to each other but incompatible to either fire or air.

Both the masculine and femininie genders are conflicting towards one another but equal in strength and this must be so in order to preserve balance in the universe.

Planets in their own signs(dignity) are strongest and purest and seek one another out as kings seek kings or presidents seek presidents. There are twelve such positions and are found in the charts of highly gifted or powerful individuals. They are:

Sun in Leo, Moon in Cancer, Mercury in Gemini, Venus in Taurus, Mars in Aries, Jupiter in Sagittarius, Saturn in Capricorn, Uranus in Aquarius, Neptune in Pisces, Pluto in Scorpio, Earth/Ascendant in Libra, and an as yet undetermined body or planet in Virgo.

Next in power are planets in their opposite signs, trine signs, square, sextile, and inconjunct or semi-sextile, and they too will primarily seek one another out within their own group or level.

Sun in Aquarius, for example, is an opposition placement(the planet and sign are opposing in quality) and will primarily seek out other opposition placements, such as Mars in Libra, Venus in Scorpio, Uranus in Leo(mutual reception), and so on.

In this way every one of the 144 planetary placements in astrology(12 planets x 12 signs) will have eleven other placements that are of primary importance to them. These placements can be grouped as follows...(select any planetary placement within any group below and see which other placements are most closely associated with it).

Example: the most important or significant placements for a Sun in Leo are Moon in Cancer, Mercury in Gemini, etc. For a Pluto in Scorpio they will be of the same group(Sun in Leo, Moon in Cancer, etc.)

Planets in Their Home Sign
Sun in Leo	Moo in Can	Mer in Gem	Ven in Tau	Mar in Ari	Jup in Sag	Sat in Cap	Ura in Aqu	Nep in Pis	Plu in Sco	Asc in Lib	X in Vir

Planets in Their Lower Semi-Sextile Sign
Sun in Vir	Moo in Leo	Mer in Can	Ven in Gem	Mar in Tau	Jup in Cap	Sat in Aqu	Ura in Pis	Nep in Ari	Plu in Sag	Asc in Sco	X in Lib

Planets in Their Lower Sextile Sign
Sun in Lib	Moo in Vir	Mer in Leo	Ven in Can	Mar in Gem	Jup in Aqu	Sat in Pis	Ura in Ari	Nep in Tau	Plu in Cap	Asc in Sag	X in Sco

Planets in Their Lower Square Sign
Sun in Sco	Moo in Lib	Mer in Vir	Ven in Leo	Mar in Can	Jup in Pis	Sat in Ari	Ura in Tau	Nep in Gem	Plu in Aqu	Asc in Cap	X in Sag

Planets in Their Lower Trine Sign
Sun in Sag	Moo in Sco	Mer in Lib	Ven in Vir	Mar in Leo	Jup in Ari	Sat in Tau	Ura in Gem	Nep in Can	Plu in Pis	Asc in Aqu	X in Cap

Planets in Their Lower Quincunx Sign
Sun in Cap	Moo in Sag	Mer in Sco	Ven in Lib	Mar in Vir	Jup in Tau	Sat in Gem	Ura in Can	Nep in Leo	Plu in Ari	Asc in Pis	X in Aqu

Planets in Their Opposing Sign
Sun in Aqu	Moo in Cap	Mer in Sag	Ven in Sco	Mar in Lib	Jup in Gem	Sat in Can	Ura in Leo	Nep in Vir	Plu in Tau	Asc in Ari	X in Pis

Planets in Their Upper Quincunx Sign
Sun in Pis	Moo in Aqu	Mer in Cap	Ven in Sag	Mar in Sco	Jup in Can	Sat in Leo	Ura in Vir	Nep in Lib	Plu in Gem	Asc in Tau	X in Ari

Planets in Their Upper Trine Sign
Sun in Ari	Moo in Pis	Mer in Aqu	Ven in Cap	Mar in Sag	Jup in Leo	Sat in Vir	Ura in Lib	Nep in Sco	Plu in Can	Asc in Gem	X in Tau

Planets in Their Upper Square Sign
Sun in Tau	Moo in Ari	Mer in Pis	Ven in Aqu	Mar in Cap	Jup in Vir	Sat in Lib	Ura in Sco	Nep in Sag	Plu in Leo	Asc in Can	X in Gem

Planets in Their Upper Sextile Sign
Sun in Gem	Moo in Tau	Mer in Ari	Ven in Pis	Mar in Aqu	Jup in Lib	Sat in Sco	Ura in Sag	Nep in Cap	Plu in Vir	Asc in Leo	X in Can

Planets in Their Upper Semi-Sextile Sign
Sun in Can	Moo in Gem	Mer in Tau	Ven in Ari	Mar in Pis	Jup in Sco	Sat in Sag	Ura in Cap	Nep in Aqu	Plu in Lib	Asc in Vir	X in Leo

All planets(except for those in their home sign) are dispositing planets; that is, they are reliant or dependant on other planets in the chart(including other people's charts) for self-completion or fulfillment. Disposition works through rulership. Sun in Aries, for example is disposited through Mars, which may in turn be disposited through another planet. It is possible to have a chain of dispositing planets such as the following:

Sun in Aries/Mars in Gemini/Mercury in Taurus/Venus in Aquarius/Uranus in Sagittarius/Jupiter in Leo/Sun in Aries. In such a case, all of these planets work together in some way.

A planet in its own sign stands alone and can freely give or refuse to give of itself to the rest of the chart. Even the transits and progressions of other planets to it don't have much effect. A planet in it's own sign is a MASTER planet and needs no one or nothing(except other master planets). Dispositing planets, on the other hand, reflect all of our needs and interdependencies.

Two planets in mutual reception are the next best thing to a planet in its home sign, especially if they are of the same element or polarity. Two planets are in mutual reception when they disposit each other.

example 1: Sun in Aries and Mars in Leo

Each planet is in the sign the other rules so that there is a cross-link or interdependancy. Both planets complete or fulfill each other, and in a positive way, since both are trine aspect(by sign).

example 2: Venus in Aquarius and Uranus in Taurus

These two planets wil also fulfill or complete each other but it won't be easy as the two are in square(by sign).

Mutual reception can also work by transit or between planets in two people's charts. If you are a Sun in Aries, for example, you will be especially sensitive to the transit of Mars in Leo, or to someone's Mars in Leo.