A set is a collection of objects that have something in common or follow a rule. The objects in the set are called its elements. Curly braces are used to indicate that the objects written between them belong to a set. It is not necessary to list every object in the set. For example, the numbers 2, 4, and 6 are distinct objects when considered separately, but when they are considered collectively, they form a single set of size three, written {2,4,6}. The collection of students of a class with heights between 135 cm and 160 cm is well-defined, so it forms a set. Sets are well-determined collections that are completely characterized by their elements. Thus, two sets are equal if and only if they have the same elements. The basic relation in set theory is that of element hood or membership