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In major Western Ceremonial Magick traditions, the altar is described as being a "double cube." That is, it had two equal-sized cubes on top of each other. So if your altar was to be 36" tall, each side would be 18". That's the way I made my first altar. I painted it completely white (representing the spiritual light) and then decoupaged a large pentagram to the top. On one side I decoupaged a poster of the Tree of Life. I used this for many years before giving it to a temple I had been working with.
My current ceremonial magick altar is also a double cube, however it is painted black. This is to symbolize the concept that our physical universe, where I do magickal work, is spiritually