Like slide rules, adding machines date back to the 17th century. They started with simple machines that could only add (and sometimes subtract.) Many were rather tricky to use and could produce erroneous results with untrained users.
Apparently, Wilhelm Schickard produced the first adding machine in 1623. Unfortunately, this one-of-kind machine was destroyed in a fire and its existence remained unknown until recently.
Blaise Pascal (re)invented an adding/subtracting machine in 1642 with no knowledge of Shickard's machine. Pascal made many of his machines and is therefore often thought of as the original inventor. His first machine was 14" x 5" x 3" and had 8 digits. The closest machine shown here to Pascal's is the Addometer shown in the Simple Dial Or Slide Adders section.