Chart of a Finnish politician, Placidus houses

These are a category of House Systems (also called unequal house systems,) because they normally give rise to houses encompassing varying numbers of degrees; although each house will have the same number of degrees as the house opposite to it (for example, the second and eighth houses). In contrast, a whole sign or Equal house system consistently yields houses of 30 degrees each.

Differences in Calculation

In a quadrant house system, the chart is divided into quarters by the Medium Coeli and Imum Coeli (midheaven or MC and IC) axis and by the Ascendant and Descendant axis. These lines form the cusps of the first, fourth, 7th, and 10th houses. In contrast, in an equal house or whole sign system, MC and IC will normally fall somewhere within their four houses. Examples of popular quadrant house systems are Placidus and Koch for birth charts, and the Regiomontanus system in horary astrology.

Same person, Equal houses[1]

Although all house systems in use today involve a projection of a 12-fold division of the cosmos onto the Ecliptic the various quadrant house systems differ in terms of several key concepts important in conceptualizing and dividing the three-dimensional space in which planets, stars, and abstract points move over the course of a day. House systems vary in terms of which reference points are used to bisect the Celestial Sphere and whether the house divisions represent measurements of segments of time or of distance.

For example, one could determine the “up-down” axis of a chart in terms of the highest point of the sun’s daily journey (the MC) and its opposite point (the IC) or as the highest point in the sky (the Zenith) and its opposite point, the Nadir. The ascendant/descendant axis might be calculated as the simple visible horizon positions of the sun at dawn and sunset, or as a projection of the earth’s equator outward onto the celestial sphere, the Celestial Equator. In visualizing the course of the sun from its point of dawn in the east to its highest point in its journey across the sky, one could measure the sun’s course in units of time (hours and minutes) or in units of distance, normally calculated as degrees.