It originates from the 3rd century BCE and divides the four quadrants in three equal segments. There are numerous unequal house systems. However, the houses situated opposite each other are always the same size. the four segments defined by the main axes, are divided into three further segments of unequal size. In the unequal house systems the four quadrants, i.e. There, the sign of the Ascendant is the first house, with the Ascendant somewhere in it. The Whole Sign house system is the most ancient house system of all. Intercepted signs do not occur in equal house systems. The Medium Coeli, on the other hand, does not have to be in the tenth house but can be situated anywhere between the eighth and the eleventh house. Here the zodiac is divided into equal segments of thirty degrees with the Ascendant either marking the cusp of the first house or situated within it. In most cases, the Ascendant marks the cusp of the first house and the Medium Coeli the cusp of the tenth house. However, no matter which system is used, the axes Ascendant/ Descendant and Medium Coeli/ Imum Coeli stay the same. There are both so-called equal and unequal house systems. The houses form the horoscope's 'terrestrial' frame of reference, as opposed to the zodiac which forms the 'celestial' frame of reference. The system used to divide the zodiac into twelve further segments called houses (see diagram).
