RPL (derived from Reverse Polish Lisp according to its original developers, whilst for a short while in 1987 HP marketing attempted to coin the backronym ROM-based Procedural Language for it.) is a handheld calculator operating system and application programming language used on Hewlett-Packard's scientific graphing RPN (Reverse Polish Notation) calculators of the HP 28, 48, 49 and 50 series, but it is also usable on non-RPN calculators, such as the 38, 39 and 40 series.RPL is a structured programming language based on RPN, but equally capable of processing algebraic expressions and formulae, implemented as a threaded interpreter.