In astrology the concept of derivative houses involves starting or measuring a system of twelve houses from some point in the chart other than the Ascendant, in order to study matters relative to the person or topic signified by the point you started from.
Normally the 12 houses are measured from the Ascendant, but derivative houses involves using other points in the chart to serve the same role, as an alternate Ascendant to calculate an alternate set of houses from.
Derivative houses are usually measured from a specific house or planet in the chart, although they are also sometimes measured from certain Lots or Arabic Parts, such as the Lot of Fortune.
The process usually involves symbolically turning the chart so that the point that you want to start from becomes the Ascendant or first house, and then the other houses are determined relative to that point, in zodiacal order.
Derivative Houses from Houses
Derivative houses are usually applied to other houses in a chart order study a topic as if you were looking at it from the perspective of the person associated with that house.
For example, the 7th house is said to signify the spouse or marriage partner in the chart. When using derivative houses, the 7th house becomes the 1st house, the 8th house becomes the 2nd house, the 9th house becomes the third house, and so on.
These houses are then interpreted relative to the person associated with the house you started from. So, for example, the 2nd house from the Ascendant is usually associated with the native’s finances. By extension, the 2nd house relative to the 7th house is associated with the finances of the marriage partner, using derivative houses. Again, the 3rd house from the Ascendant is associated with the native’s siblings, and by extension the 3rd house from the 7th is associated with the partner’s siblings.
The earliest list of significations for using derivative houses appears in the work of the 2nd century astrologer Vettius Valens, although it appears to based on an earlier work attributed to Asclepius.
Derivative Houses from Planets
Derivative houses can also be done from specific planets, so that the planet becomes an alternative Ascendant in the chart. This approach has been used in a few different traditions of astrology.
Many modern writers of horoscope columns that are based on Sun-signs use derivative houses from the sign of the natal Sun in order to add some additional interpretative details. In this approach the sign that contains the Sun becomes the first house, the sign after the Sun becomes the 2nd house, the next sign becomes the 3rd house, and so on.
In India astrologers will commonly calculate an alternative set of whole sign houses from the sign of the Moon, which is known as the Chandra Lagna, which means “Moon Ascendant”.
Derivative Houses from Lots
In the Hellenistic tradition astrologers would commonly use the Lot of Fortune or Part of Fortune as an alternative Ascendant, and calculate a set of derivative whole sign houses from the sign that it was in.
Within the context of certain timing techniques such as Zodiacal Releasing some astrologers such as Vettius Valens said that the houses and especially the angles measured from the Lot of Fortune were more important and more powerful than the houses measured from the Ascendant.