Each sign is affiliated with a planet said to be its ruler. The ruling planet expresses its nature in the purest and strongest manner in the sign it rules. Both the sign and any house(s) with the sign sitting at the cusp (doorway) are impacted by the “condition” of the sign’s ruling planet. (Strength or weakness is determined according to its sign and house positions, direction and speed of motion, aspects, etc.) The sign and its governing planet together are said to rule (share a strong association with) a particular, and wide-ranging, set of: circumstances and conditions; creatures and categories of people; various behaviors; and a host of nouns, adverbs and adjectives. A good resource for this is The Rulership Book by Rex E. Bills (commonly referred to by astrologers as Bills’ book of rulership).
The Signs and their Ruling Planets
Scorpio Pluto (traditional ruler Mars)
Aquarius Uranus (traditional ruler Saturn)
Pisces Neptune (traditional ruler Jupiter)
Some planets rule one sign; some, two. Before the discovery of planets you need a telescope to see, the traditional ruling planets were those visible without even binoculars: Mercury, Venus, Mars, Jupiter and Saturn. Each ruled two signs. The luminaries, the Moon and the Sun, ruled one a piece. Observations after the discovery of Uranus, Neptune and Pluto, led to assignment of their modern rulership of Aquarius, Pisces and Scorpio, respectively. Only Mercury and Venus currently rule two signs.
In working with a chart, you can play “follow the ruler.” The ruler of your rising sign (Ascendant) rules your whole chart. Look for the ruling planet of your rising sign. It is said to be the dispositor of that sign. What sign is it in? Then, what rules that sign, where is it? When a planet is in the sign it rules, that’s the end of the line. When you follow the ruler for all your natal planets, if they all lead to one planet in the sign it rules, that’s called your sole dispositor and its role in your life is very important. You might have more than one planet in the sign it rules, thus no sole dispositor, rather two or three planets which are still quite important. Two planets can be in the sign each other rules, called mutual reception, supporting each other’s indications. Maybe a trio of planets disposit one another in a loop. These types of combinations may be where the buck stops, and their interwoven influences impact your life noticeably.