Comptometer
The comptometer , invented by Dorr Felt in 1887 , was the first mechanical calculating machine to be operated solely by pressing keys. Each decimal place had a row of nine keys with values from 1 to 9. Although the comptometer was mainly designed for addition and subtraction , it could also do the other basic arithmetic operations . All decimal places of the numbers to be processed could be entered simultaneously or one after the other. Due to the simultaneous entry, the numbers could be entered much faster compared to the sequential entry in today's pocket calculators.
There were comptometers for special applications such as currency calculations; these versions could have between 30 and 100 keys.
In some areas, comptometers were still in use until the 1990s.
