Reverse Polish notation (RPN) is a mathematical notation in which every operator follows all of its operands, in contrast to Polish notation, which puts the operator in the prefix position. It is also known as postfix notation and is parenthesis-free as long as operator arities are fixed.ewlett-Packard engineers designed the 9100A Desktop Calculator in 1968 with RPN with only three stack levels, a RPN variant later referred to as three-level RPN. This calculator popularized RPN among the scientific and engineering communities, even though early advertisements for the 9100A failed to mention RPN.