The great majority of astrologers use a system of 12 houses, where the first house is the area of the sky just below the eastern horizon, and the houses are numbered counterclockwise. When the sun rises in the morning it enters the 12th house and travels backwards through the 12th house for approximately 2 hours and then it enters the 11th house. About 4 hours after sunrise, the Sun enters the end of the 10th house and at midday (approximately 12 noon) it enters the end of the 9th house, and so on around the chart wheel. At sunset the Sun leaves the 7th house and enters the end of the 6th house. At approximately 12 midnight the Sun leaves the 4th house and enters the end of the 3rd house.
The dividing line between one house and another is called the house cusp, so, for example, when the Sun moves from the 10th house to the 9th house around 12 noon, it is crossing the 10th house cusp.
There are some alternative ideas about the positions of houses: for example, some astrologers believe that the house cusps are the center of the house and not the beginning of the house. An even more radically different (and unusual) idea is that there are actually only 8 houses and not 12. In Chapter 19 of the book Astrological Origins, Cyril Fagan, for example, proposes the existence of a house system based on an 8 house division.
The most important house cusps are the 1st, 4th, 7th, and 10th. Almost all astrologers agree that a planet near one of these house cusps is very powerful. These 4 house cusps are sometimes referred to as the angles or the angular house cusps. The other cusps (the 2nd, 3rd, 5th, 6th, 8th, 9th, 11th, and 12th) are sometimes referred to as the intermediate cusps. Astrologers generally agree about the positions of the angles, but there is a great deal of disagreement about precisely where the intermediate cusps are. Many different mathematical formulae have been proposed to calculate where the intermediate cusps are. The various house systems such as Campanus, Porphyry, Koch, Placidus, etc. differ in their determination of where the intermediate cusps are located.