Quick-name the first personal computer. No, it wasn't the 1981 IBM PC, which introduced "PC" to the vernacular. Nor was it the 1977 Apple II. Not even the 1975 Altair-the first machine to run Microsoft software-represents the true origin of this species.

The first digital PC showed up in 1950, the creation of early computer entrepreneur Edmund C. Berkeley. Berkeley caught computer fever while working as an analyst at Prudential. He pushed for Prudential's involvement in the development and use of UNIVAC, the first general-purpose commercial computing machine. In 1948, Berkeley founded his own computer company, Berkeley Associates, in Newton, MA. There he conceived of Simon, which he first described in his 1949 book Giant Brains, or Machines That Think.

Like present-day PCs, Simon was programmable, affordable to an average professional and could be operated with the aid of only a simple training manual. Shown above with Berkeley, Simon consisted of 129 electromechanical relays, a stepping switch and a paper-tape feed for inputting data. It performed simple arithmetic and logic operations, displaying the results with an array of five lights. Twenty-five years later, the Altair didn't do much more.

Berkeley built the first Simon with a team of Columbia University students in 1950. Simon's purpose was to demonstrate the accessibility and utility of electronic computers by giving people hands-on experience. He marketed plans for Simon rather than assembled systems; more than 500 copies of the plan were eventually sold. The machine, which cost $300 to $600 to build (equivalent to about $2000 to $4000 today), helped illustrate basic computing concepts without huge machines that only a few elite institutions could afford.