Derived houses (also known as bhavat bhavam in vedic astrology) in tropical, western astrology is an astrological theorem or principle to horoscope analysis.
This theorem states that any house in a horoscope inherits the traditional, root meaning of a house based on the origination from which houses have been counted.
This means that in a general horoscope, the 12 houses have their base or root meaning and significations, but additional meanings and significations are added on top of the root meanings when the horoscope is analyzed from different “view points” or origination points. This layered meaning reveals background information to give more details and support to the horoscope's story.
Let's take a look at a couple visuals to help illustrate the concept.
Example #1: Father's Spiritual & Religious Beliefs
In the illustration above, we can see that the traditional house count begins at house #1 just below the eastern horizon. But, let's say we want to learn more about the 4th house's affairs, specifically, since the 4th house represents the father in a natal chart, we want to learn more about him.
We can use the 4th house as an origination point to tell us more information about the father or other 4th house associated affairs or people. So, beginning at the natal root 4th house, we'd count the natal root 4th house as the 1st derived house from the natal root 4th house, natal root 5th house as the 2nd derived house from the natal root 4th house, natal root 6th house as the 3rd derived house from the natal root 4th house, and so on and so forth until we reach the natal root 12th house. The natal root 12th house is the 9th derived house from the natal root 4th house.
This end result translates as the natal root 12th house providing additional background information into the father's spirituality, philosophies, religious beliefs, and higher education.
Example #2: Mother's Siblings
To continue is illustrating the concept, we can look at example two. In this example, we start counting houses from the natal root 10th house because we want to get more background information about the native's mother, as she is represented by the 10th house.
The natal root 11th house is the 2nd derived house from the natal root 10th house and the natal root 12th house is the 3rd derived house from the natal root 10th house.
This end results translates as the natal root 12th house providing more insight into the mother's siblings.