Cartesian Products
Then the cartesian product S1 X S2 is the set of all ordered pairs in which the first element is a member of S1 and the second element is a member of S2.
Let A = { a, b, c }, Let B = { 1, 2 } then
A X B = { (a, 1), (a, 2), (b, 1), (b, 2), (c, 1), (c, 2) }