The calculator that killed the slide rule. Bill Hewlett challenged his engineers: fit a scientific calculator into a shirt pocket. They delivered the HP-35—35 keys, all scientific functions, in a 9-ounce package.
It used Reverse Polish Notation (RPN): enter numbers, then the operator. No "=" key needed. The 4-level stack (T, Z, Y, X) made complex calculations elegant.
Price: $395 ($2,800 today). HP expected to sell 10,000. They sold 300,000 in the first three years. Keuffel & Esser, the largest slide rule maker, ceased production.
RPN: type number, press ENTER, type next, press operator • Try: 2 ENTER 3 + = 5