Reverse Polish Notation uses a system that involve "stacks", each of which stores a certain value in specific ranks. The stacks usually start off with the first stack (often called the X stack), followed by the second stack (often called the Y stack), the third stack, and so on. New values can be entered on the first stack by pushing up the stack ranks by one. On a Reverse Polish Notation calculator, the "Enter" function would do the same. 