A set is a collection of unique objects called elements. A set is usually written within curly brackets; the order of the elements in the set does not matter. The example above is a list of objects, but sets can be made up of numbers as well! Notice that although occurs twice in the list, unique elements of a set are only considered once and the original order of the list can be disregarded. Generally speaking, a number set is a group of numbers that share certain characteristics. When discussing mathematical theory, it is important to understand the distinction between certain number sets.