9100A Desktop Calculator, 1968
Really a desktop computer, the 9100A combined Reverse Polish Notation (RPN) — a system for representing mathematical expressions without the use of parentheses — with a special algorithm that could handle trigonometric and logarithmic functions. HP sold the 9100A as a calculator because at the time the perception was that a computer had to be big to be credible. Bill Hewlett said "If we had called it a computer, it would have been rejected by our customers' computer gurus because it didn't look like an IBM. We, therefore, decided to call it a calculator and all such nonsense disappeared."
The 9100A is now recognized throughout the industry as the first desktop computer. In December 2000, Wired Magazine named it the first personal computer.