QWERTY
History
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.
QWERTY-5