Keyboard layout and data entry
Answers
The construction of the "Type Writer" had two flaws that made the product susceptible to jams. Firstly, characters were mounted on metal arms or type bars, which would clash and jam if neighbouring arms were pressed at the same time or in rapid succession. Secondly, its printing point was located beneath the paper carriage, invisible to the operator, a so-called "up-stroke" design. Consequently, jams were especially serious, because the typist could only discover the mishap by raising the carriage to inspect what had been typed. The solution was to place commonly used letter-pairs (like "th" or "st") so that their type bars were not neighbouring, avoiding jams.[2]
Sholes struggled for the next five years to perfect his invention, making many trial-and-error rearrangements of the original machine's alphabetical key arrangement. The study of bigram (letter-pair) frequency by educator Amos Densmore, brother of the financial backer James Densmore, is believed to have influenced the array of letters, but the contribution was later called into question.[3] Others suggest instead that the letter groupings evolved from telegraph operators' feedback.