That’s a photo of the HP-35 calculator, predecessor of my beloved HP-11C for scientific calculations (and 12-C too, for financial calculations). Both used Reverse Polish Notation, or Reverse Polish Prefix as I recall, which is the BEST.

Reverse Polish

The HP Museum has a nice page about RPN. It describes the difference between algebraic and RPN calculators, as well as the story behind HP’s decision to use RPN for the first HP calculators.

Once the technology to produce algebraic compilers could fit into a pocket calculator, most RPN users had decided that RPN was more efficient and consistent for the user as well as for the calculator.

I definitely agree with that! This too:

Also, because subexpressions are evaluated as they are entered, entry errors are more obvious with RPN. On an algebraic calculator, omitting an opening parenthesis, may not lead to a calculation error until much later when an entire subexpression is evaluated.

Even if you don’t read all of it, make sure to scroll down to the end of the page, and have a look at those very cute GTO radio buttons.

Once and Future HP

Surprising how contemporary, androgynous, and attractive, the woman in this advertisement was. Photograph date is approximately 1973. It was the lead-in to a Wired Magazine article I saw this morning.

The primary subject of the article is okay, but the intro, a lengthy section detailing the history of HP products, was better:

In 1971, Hewlett-Packard commissioned a marketing study to see if anyone would buy a $395 pocket calculator. The marketer’s verdict? Make it the size of a typewriter, because nobody wants a small machine.

Luckily, Bill Hewlett ignored the marketers, and the next year, the company introduced its HP-35 Scientific Calculator. The pocket calculator was a hit, eventually making slide rules obsolete.

— After Cuts, HP Labs Vows Return to Glory Days,15 February 2012