Each house system uses a different set of mathematical formulae to calculate the cusps. The Porphyry system of houses is an easy one to understand; in this house system the angle between the Ascendant and MC is divided into 3 equal sections.
In the Porphyry system, the 1st, 2nd, 3rd, 7th, 8th, and 9th houses are all the same size. The other 6 houses also have the same size. Another house system that is simple is the equal house system where each house has a size of 30 degrees. In the equal house system, the MC is a sensitive point that is used but it is not the beginning of the 10th house as it is in most other house systems. The other house systems use various more technically complex means to divide the sky such as dividing the celestial equator, or horizon into equal sections and then projecting these points on to the ecliptic plane.
Two house systems are referenced in the ancient material. The first takes the zodiacal sign of the Ascendant and considers that whole sign to be the First house. Houses and Signs were used interchange ably. This usage is called "Whole Sign" houses. The second method takes the Ascendant degree and subtracts five degrees from it (which places you into the modern Twelfth house). That point (which is called the horoskopos) then becomes the cusp of the First house in this system, and all other houses are measured thirty degrees from the horoskopos. Note that the horoskopos is not the Ascendant, nor does it replace the Ascendant. Houses are also referred to as Places, or Domiciles.
In Vedic Astrology Houses and signs are used interchangeably. Whatever sign is on the Ascendant becomes the 1st house. The sign is the house, the house is the sign. The Ascendant then is a point which lies in that 1st house, but the cusp of the 1st house is zero degrees of the sign which the Ascendant occupies, in the Rasi or natal chart. The Porphyry house system (see "House Systems") is commonly used in Vedic astrology
|1st:||Bhava Lagna, Kalpa, Tanu, Udaya, Vilagna|
|2nd:||Artha, Dhana, Nyana, Swa, Vak, Vitta|
|3rd:||Dhairya, Dushchikya, Sahodara, Veerya, Vikrama|
|4th:||Ambu, Geha, Hibuka, Jala, Matru, Rasatala, Sukha, Vidya, Vriddhi|
|5th:||Deva, Dhee, Nandana, Panchaka, Pitru, Putra, Santana|
|6th:||Bhaya, Ripu, Roga, Shashta, Shastra, Shatru|
|7th:||Asta, Dyuna, Gamana, Jamitra, Kama, Saptama|
|8th:||Mrityu, Parabhava, Rana, Randhra, Vinasha|
|9th:||Dharma, Nava, Shubha, Tapu|
|10th:||Aspdad, Gnana, Karma, Kha, Madhya Lagna, Meshurana, Vyapara|
|11th:||Aya, Labh, Upantya|
|12th:||Antya, Dwadashaka, Rina, Vyaya|