Howard Aiken and Grace Hopper designed the MARK series of computers in Harvard University. The MARK series of computers began with the Mark I in 1944. Imagine a giant room full of noisy, clicking metal parts, 55 feet long and 8 feet high. The 5-ton device contained almost 760,000 separate pieces. Used by the US Navy for gunnery and ballistic calculations, the Mark I remained in operation until 1959.

The computer, controlled by pre-punched paper tape, could carry out addition, subtraction, multiplication, division and reference to previous results. It had special subroutines for logarithms and trigonometric functions and used 23 decimal place numbers. Data was stored and counted mechanically using 3000 decimal storage wheels, 1400 rotary dial switches, and 500 miles of wire. Its electromagnetic relays classified the machine as a computer of relay. All output was displayed on an electric typewriter. By today's standards, the Mark I was slow, requiring 3-5 seconds for multiplication.