A parallel universe is created in the Ethelverse each time a choice is made. A universe is created in which the final descision was not the one taken, and the future is altered. Although this can sometimes be as simple as what color pen to use, it can also be as vital as the descision whether or not Columbus sailed the ocean.
A universe can also be created if the universe is the product of supernatrual forces, such as a wish or spell.
Becuase of the simplicity of creating a parallel universe, a near infinite number exist.
Visiting a parallel universe is difficult, but not impossible. There are a few artifacts with the ability to do so, as there are some spells as well. Some such artifacts include the Hyperion Cube and the Romidius.
The primary universe in the Ethelverse, The Base, was the original universe, until the first choice was made. The Base is also the only universe in which a parallel universe can be made from. For instance, a choice in a parallel universe would not create another parallel universe. The creation can only be the result of choices made in The Base.
This also means that anybody existing at any given moment is also in as parallel universes as choices are made at the moment. These alternate choices, however, can lead to a different furture, different people existing/dying, and a much different version of that person.