The principal concept of set theory, the one that in completely axiomatic studies is the principal primitive (undefined) concept, is that of belonging. If x belongs to A (x is an element of A, x is contained in A), we shall write xEA.
A possible relation between sets, more elementary than belonging, is equality. The equality of two sets A and B is universally denoted by the familiar symbol A =B; the fact that A and B are not equal is expressed by writing A**≠**B.