Thomas de Colmar arithmometer, made in around 1875. Colmar introduced his first version in 1820 and then made many improvements to what became the first commercially successful calculating machine. The design was not sufficiently reliable for it to come into common usage until the late 19th century. Despite the improvements, there was still a risk of error in the transcription of the result and the operator needed to understand the mathematical principles involved. The arithmometer is based on the Leibniz stepped reckoner, or stepped drum.