The subscript gives the symbol for a bound variable ("i" in this case), called the "index of multiplication", together with its lower bound ("1"), whereas the superscript (here "4") gives its upper bound. The lower and upper bound are expressions denoting integers. The factors of the product are obtained by taking the expression following the product operator, with successive integer values substituted for the index of multiplication, starting from the lower bound and incremented by 1 up to (and including) the upper bound. For example: