Altars can be found in every church, spiritual sight, and most homes (even when it is not considered or recognized a such. Such as the pictures of relatives that have passed on the fireplace mantle). Depending on the intended use and culture or practice the Altar is found in conjunction with is going to depend on what type of items that may be on it. Some of the most common items are ritual tools, statues or representations of Deity and Spirits, divination objects, images or representations of ancestors, herbs, minerals, artwork, leaves, etc. The list can really go on and on. Some people have one Altar that serves many different functions, some have an separate alter for their work and their ancestors, other people may have seasonal Altars, and some people treat their Altars as something that is fluid as they add or remove various items (in addition to possibly changing the use or meaning of the Altar itself).
It is beneficial to have different Altars dedicated for different purposes. This allows the energy to stay focused in addition to the mind when someone is making use of said Altar.
In various traditions and systems (for example: Traditional Witchcraft) it is a common theme to find three different Altars suggested and discussed.
The first Altar being the Genius Loci. This Altar is usually located outside close to the person's house. Some of the main functions of this is to be a sacred area to leave offerings for spirits and Deity, a place to bless objects, a focus point for outdoor ritual, and to act as housing for the local land and house wights. In Heathen circles, this is known as a Harrow or Horg and is commonly made of stone.
The second Altar being the Ancestral Altar. This is usually located inside the home and its main focus is for Ancestral veneration. Usually pictures of ancestors, items that represent them, and even some of the ancestors personal belongings can be found on these Altars. The style of veneration various from person to person, culture to culture, and tradition to tradition. Though common threads do exist such acting as a place to leave offerings or messages, to ask for their wisdom, and as a sign of respect.
On my personal Altar there is a Hearthstone (it represents strength and foundation), a Skull (is a memento mori, represents our connection to the dead, and serves as housing for an ancestral spirit during ritual), and a message box (it's a wood box where I leave messages for my ancestors when I wish to communicate. During the full moon I burn any messages that may have been transmitted).
The third Altar being the Sacrificial Altar. This is the work horse of the Altars and is usually found inside the house. It is where a lot of ritual, meditation, prayer, and Magick is conducted. On it one can usually find ritual tools, herbs, depictions or representations of various spirits and Deities, candles, oils, divination objects (such as Tarot Cards, Runes, etc), and so on. There is no limitation to what can be found on a Sacrificial Altar and what can work is done with it. It is also something that can be moveable (and is usually considered to be more so then the Ancestral and Genius Loci Altars) depending on where the ritual is going to be.
There is a fourth Altar that is rarely if ever talked about, discussed, printed, and taught outside of the passing warning about not participating in ritual or conducting Magick if you are physical ill or mentally/emotionally off. This is the Altar inside of the person, their Divine Spark, and their mind. Outside the practitioners of Vedic derivities most people are so focused on conducting outward Magicks that they neglect their Altar inside of themselves. It is more important then anything else to focus on introspection, build that inner Altar, and keep it purified as much as possible. The person is the intent, the will, the conduit, and the lightning rod. An unstable person no matter how intense their focus, how precise their incantations, how elaborate their rituals and Altars, will never achieve the desired effects due to their malleable foundation.
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