 Forth relies heavily on explicit use of a data stack and reverse Polish notation ( RPN or postfix notation ) , commonly used in calculators from Hewlett @-@ Packard . In RPN , the operator is placed after its operands , as opposed to the more common infix notation where the operator is placed between its operands . Postfix notation makes the language easier to parse and extend ; Forth 's flexibility makes a static BNF grammar inappropriate , and it does not have a monolithic compiler . Extending the compiler only requires writing a new word , instead of modifying a grammar and changing the underlying implementation . 
