HP-35

The HP-35 was Hewlett-Packard's first pocket calculator and the world's first scientific pocket calculator (a calculator with trigonometric and exponential functions). Like some of HP's desktop calculators it used reverse Polish notation. Introduced at US$395, the HP-35 was available from 1972 to 1975.

Market studies at the time had shown no market for pocket sized calculators. However, HP co-founder Bill Hewlett began development of a "shirt-pocket sized HP-9100", and it turned out that the marketing studies were wrong. In the first months orders were exceeding HP's expectations as to the entire market size. Before the HP-35, the only practically portable device for performing trigonometric and exponential functions were slide rules. Existing pocket calculators at the time were only four-function, i.e., could only do addition, subtraction, multiplication, and division.