Phi are the spirits of buildings or territories, natural places, or phenomena. They are also ancestral spirits that protect people, or can also include malevolent spirits. The phi which are guardian deities of places, or towns are celebrated at festivals with communal gatherings and offerings of food. Many Lao will have a spirit house on or near their property which is an important folk custom used to ensure balance with the natural and supernatural world. Phi were believed to influence natural phenomena including human illness and so appealing to the phi became an important part of Lao identity and religious health over the millennia. Astrology was a vital part to understanding the natural and spiritual worlds and became an important cultural means to enforce social and customs.