Here is where the story gets interesting. Why did Felt choose to display the numbers in a 9 to 1 sequence? It wasn’t a widespread notion at the time. After all, the knowledge of arithmetical devices was not as widespread.
A reasonable answer could be tied to some mechanical decisions, possibly related to the method of complements and the fact that keys were pressing levers connected to rotating drums (Durant, 2011). The longer stick was equal to a longer rotation, which meant to be the number 9, as opposed to number 1 which required a shorter rotation — a suggestion from an older concept by Parmelee.
Another interesting explanation — from a modern design standpoint — goes beyond the mechanical reasons. According to the Comptometer Manual, operators were meant to input numbers by using the lowest values on the keyboard. For instance, in order to enter “9 cents”, the operator was not supposed to press the 9 key at the very right column. Instead, they were to press, in sequence, the 4 and 5 key. The machine would do the math. Reaching for the “9” key was deprecated because it decreased the figuring speed if users had to move their right hand from the bottom. Felt was all about efficiency, which meant keeping commonly used keys within the fingers’ reach. It seems this need for efficiency led to this user-centered design layout, but it was still not considered a user-friendly interface (Meehan, 1952).
The Comptometer and its competitors required highly-trained users to attain maximum productivity. It was also difficult to do with one hand, especially when it came to multiplications.
In 1902, the Dalton went on to become one of the most popular 10-key adding machines of the time, rendering multi-column calculators obsolete. Dalton was a miniature version of a typewriter and had two rows of five buttons with an odd arrangement — 24579 at the top and 13068 at the bottom. What’s different in the arrangement that was not seen up until now? (Durant, 2011)
That’s right! The 0 finally appeared in a sequence.
The Dalton was a major improvement, combining the printer and calculator into a smaller size and adding a new kind of keyboard that went beyond the literal arrangements for decimals. Bookkeepers around the world rejoiced when the development of the Dalton (Dalakov, 2018).
The quest for further development continued.
In 1914, David Sundstrand, a Swedish-born American man, filed patent №1198487 under the name Sundstrand Corporation. The goal was to push the usability of these adding machines further. He rearranged the key in a more “logical, natural configuration.” It was based on a 3×3 layout, beginning with 789 at the top and a larger 0 at the bottom. It could be operated with one hand, which made it “the fastest keyboard of all adding machines.”
The layout became the standard for calculator keypads — even 100+ years later.
