Reiki is a method of working with energy that can be used for your own benefit or, increasingly, for the benefit of others. We say increasingly because Mikao Usui, the founder of Reiki intended it as a system for working on you for self-healing and spiritual development.
Reiki teaching has encouraged the use of Reiki as a treatment technique for working on others and Reiki is seen as a therapy. You can also learn more about akashic records meditation via online sources.
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Reiki practitioners work with several symbols. These symbols are used during treatments, and during attunements. Attunement is the connection ritual where a Reiki Master will connect the practitioner to the Reiki energy and to the Reiki symbols.
The symbols are seen as an integral part of the system, with a belief that you can only channel Reiki once you have been attuned to the symbols.
Reiki symbols are used to allow the practitioner to focus and work with different energies. There are many different ways that you can work with symbols and there is no one way that is right or wrong.
Some lineages include complex rules about what symbols to use when, and teach that symbols should be used together in a specific way to achieve specific results.
One of the most important things the Reiki practitioner can do though is just get out of the way and let the intelligent Reiki energy flow.