The Four Worlds

http://www.byzant.com/kabbalah/worlds.html

The sefirot, introduced in our discussion of the Tree of Life, can usefully be considered on their own, but Kabbalists further refine the system by viewing them as acting on or through four different levels or worlds which form a hierarchy of their own. Just as from Kether to Malkuth there is a successive movement from the abstract to the manifest, so there is in the Kabbalistic worlds from Atziluth to Assiah:

These four worlds can be considered as a linear hierarchy, each containing its own full Tree, in which Malkuth in one world becomes Kether of the world below, and Kether of one world becomes Malkuth of the world above. Malkuth is seen as the complementary fulfillment of Kether: the first is Divine Immanence, the latter Divine Transcendence.

An alternative is to view the four worlds as being expressed through a single Tree. Atziluth is expressed through the sefira Kether; Briah through Chokmah and Binah; Yetsirah through the six sefirot Chesed to Yesod; and Assiah through Malkuth. (Some prefer to view Atziluth as acting through both Kether and Chokmah, and Briah as acting through Binah alone.)

Both systems of either one or four Trees are equally valid and are used according to context, rather like the physicist viewing light as a wave of energy or a stream of particles depending on circumstance. A further outlook is that each sefira can be viewed as a whole Tree itself, giving ten Trees and one hundred sefirot. Kabbalistically these numbers can be considered equivalent as, using techniques similar to numerology, 100, 10 and 1 are fundamentally one (1+0+0 = 1+0 = 1).

The fourfold aspect given to each sefira by its existence in each of the four worlds allows application of the Kabbalah to fourfold systems such as the western elements (Fire, Air, Water and Earth), the letters of the Tetragrammaton (Hebrew Name of God: Yod, He, Vau, final He), the Jungian functions and the Minor Arcana of the Tarot. The latter is very important in the Western Kabbalah: each of the four Tarot suits contains precisely ten numbered cards (as do our modern playing cards which are derived from the Tarot), and so a Tarot deck contains a representation of the Tree in each of four worlds. The four court cards in each suit offer a further fourfold correspondence, and each of the twenty-two Major Arcana cards corresponds to one of the twenty-two letters of the Hebrew alphabet, and to one of the twenty-two paths joining the sefirot on the Tree of Life.

For use in prayer, meditation or magic, each sefira is assigned a Name of God to represent its essence in Atziluth, an Archangel for Briah, an Angelic Host for Yetsirah, and a Mundane Chakra (generally expressed through planetary forces) for Assiah.