The selection of a building site is a very important factor in Vastu Architecture. The Earth is considered a living organism in Vastu Science. The Vastu Shastras address this energy contained in the Earth as Vastu Purusha. Purusha means subtle energy that permeates the Earth and Vastu is the material body that evolved out of the energy. In abstraction, Purusha is but a subtle substance. Most importance is the orientation of the structure with respect to the cardinal points of the compass. According to Vastu Science, a network of energy lines covers the globe of the Earth. These lines are very similar to what we know as the longitude and latitude lines. The lines are conduits by which the energy concentrated at the center of the vital Earth flows out and spreads over the Earth. They are also lines of absorption of solar, lunar and stellar energies. The Earth pulsates with life and rays its energy through this grid system. The concept of the energy grid has been the ruling element of Vastu science in the domain of traditional designs pertaining to Indian architecture. The building plan is laid out with respect to an energy grid called the "Vastu Purusha Mandala". Once the designer establishes the grid, he can lay out the walls and pillars of the building on this pattern. This establishes a kind of geometric congruency or harmony with the earthly grid. If the structures grid is in harmony with the Earths grid, then the built space and the Earth respond harmoniously energetically. If there is any distortion, deviation or deflection in the grid lines of the proposed building grid and that of the Earths grid, then the built space is said to be in disharmony with the Earths energy grid system. Just as we, human beings, harbor in our heart an atom of Divine energy by which we are able to vibrate with life. The Earth also vibrates from within sending forth energy waves. The Vastu designer brings about harmony between the vibrations by implementing particular a mathematical order called Ayadi. The knowledge of Ayadi is used by the designer to determine the effects of the physical structure on the individual. The homeowner is born under the influence of a particular birth star. The building is considered to be a living being and also has a birth star. Ayadi calculations attempt to harmonize the energies created by the influence of the birth star of the person and that of the building. When these energies are harmonized the dweller experiences material prosperity and spiritual well being.