Forth

The Reverse Polish Notation (RPN) is a method for performing calculations without the need for parentheses. RPN was popularized in the ’70s and ’80s by Hewlett-Packard (HP), that used it in all its scientific and financial calculators. When using calculators from rival Texas-Instruments, which all relied on parentheses, it was easy to lose track of how many parentheses had been opened or closed, often forcing users to re-enter the entire expression from scratch. Those who used an RPN calculator didn’t have these problems, although they had to overcome a small initial learning curve to get used to the new notation.