Jan Lukasiewicz developed Reverse Polish Notation (RPN) in the 1920s. RPN is a postfix notation that removes the need for parentheses in complex expressions. Numeric values are pushed onto a stack and popped off to implement different functions. For example, you enter "4 + 5" as "4", "5", "+". A more complicated example shows how RPN avoids parentheses (see Table A). This concept is quite powerful once you get comfortable with it.