into the 1970s. Combined characters In early designs, some characters were produced by printing two symbols with the carriage in the same position. For instance, the exclamation point, which shares a key with the numeral 1 on post-mechanical keyboards, could be reproduced by using a three-stroke combination of an apostrophe, a backspace, and a period. A semicolon (;) was produced by printing a comma (,) over a colon (:). As the backspace key is slow in simple mechanical typewriters (the carriage was heavy and optimized to move in the opposite direction), a more professional approach was to block the carriage by pressing and holding the space bar while printing all characters that needed to be in a shared position. To make this possible, the carriage was designed to advance forward only after releasing the space bar. In the era of mechanical typewriters, combined characters such as é and õ were created by the use of dead keys for the diacritics (′, ~), which did not move the paper forward. Thus the ′ and e would be printed at the same location on the paper, creating é. Contemporary alternatives There were no particular technological requirements for the QWERTY layout, since at the time there were ways to make a typewriter without the "up-stroke" typebar mechanism that had required it to be devised. Not only were there rival machines with "down-stroke" and "frontstroke" positions that gave a visible printing point, the problem of typebar clashes could be circumvented completely: examples include Thomas Edison's 1872 electric print-wheel device which later became the basis for Teletype machines; Lucien Stephen Crandall's typewriter (the second to come onto the American market) whose type was arranged on a cylindrical sleeve; the Hammond typewriter of 1887 which used a semi-circular "type-shuttle" of hardened rubber (later light metal); and the Blickensderfer typewriter of 1893 which used a type wheel. The early Blickensderfer's "Ideal" keyboard was also non-QWERTY, instead having the sequence "DHIATENSOR" in the home row, these 10 letters being capable of composing 70% of the words in the English language. Properties Alternating hands while typing is a desirable trait in a keyboard design. While one hand types a letter, the other hand can prepare to type the next letter, making the process faster and more efficient. In the QWERTY layout many more words can be spelled using only the left hand than the right hand. In fact, thousands