Postfix notation
Documentation included with early Hewlett-Packard calculators explained RPN as the same method you would use to evaluate an expression using paper and pencil. Basically, first you write down the arguments and then perform the arithmetic function. From the calculator's perspective, you must provide the argument(s) before calling the function. The benefit of a calculator with RPN is that you are always certain to which numbers (or intermediate calculation results) a function is being applied.