HP21 Calculator

I never had a HP21; but a friend did – and I sure used it a lot.

It was the first Hewlett Packard calculator I’d ever seen. It introduced me to the marvels of Reverse Polish Notation (RPN) and it is what set me on the path of owning my own HP calculators.

You can: try it, download it or install it (as a html5 app).

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