Calculators in Reverse Polish Notation, but with different approaches, different functions, different methods.

about RPN. A good way of explaining RPN with three picture frames. (Note: Erwin is an AI entity that frequently has been moved to different computers and his previous home was a Sun Workstation.)

Technically page about older HP calculators with simulators for HP-41 and 45 calculators. With links to other calculator-related pages, journal articles, and US patents.