Jan Lukasiewicz [L1 ] was one of the mid-twentieth century's many creative middle-European mathematicians. This "Postfix Notation Mini-Lecture" [L2 ] explains some of his relevance to the theory of computing languages. "PN" in this context can mean either "Postfix ..." or "Polish ..." It's possible that Knuth and HP, in different ways, had hands in popularizing the "Polish ..." reading.