It is within reason that a locking action of that character would even defeat the speed of key-action that was possible on the old “Comptometer,” since an operator could overlap the key strokes in that machine to a certain extent; whereas the forced simultaneity of the cash register, if applied to the “Comptometer,” would prevent any overlapping or the depression of a second key until the first depressed key returned. The only simultaneity of key-action that could provide a means of speeding up the old “Comptometer,” or any machine of its type, was a means that would leave key-depression free as to matter of time; one that would be perfectly flexible in group manipulation, offering a complete fluidity of motion such as not to hinder the fingering of the operator. The purpose of the mechanical means employed to give simultaneity in the cash register was to lock all the keys depressed together and lock all others against depression until the former returned. The purpose of mechanical means employed in the Felt patent was to give perfect freedom of key-action, whereas formerly the key manipulation of the old “Comptometer” was restricted in the freedom of key-action, to the extent of being limited to seriatum action. The above discussion has been somewhat elaborately detailed to offset statements that simultaneity was old in the key-driven Art. There is no question as to the cash register type of inflexible simultaneity of action being old before Felt patented his flexible type of simultaneity of key-action for a key-driven calculating machine; but any statement intended to convey the idea that Felt’s contribution of the flexible simultaneity of key-action to the Art was not new, must come from ignorance of the facts or malice aforethought. [Sidenote: _Duplex Comptometer_] This flexible keyboard “Comptometer” was given the trade name of “Duplex Comptometer;” the term “Duplex” meaning that two keys could be depressed, as distinguished from the seriatum one at a time key-action formerly required. The term, however, fell short of setting forth the capacity of such action, as it was, in fact, not restricted to mere duplex-action--it was really a multiplex key-action having no limit except the lack of fingers on the part of the operator to depress the keys. The validity of these patents has been sustained in litigation. The technical scope of the mere claims has been disputed, as patent claims sometimes are; but the broad newness and importance of the practical calculative capacity achieved is beyond dispute. The recent machine called the “Burroughs Calculator” has multiplex key-action, but it did nothing to advance the practical capacity of key-driven calculating machines. [Sidenote: _Introduction of full-stroke mechanism_]