Now it may be said that the locking of other orders was old in the cash register; but let us analyze the scheme and action of both. The depression of a key of the key-driven cash register immediately locked all other keys not depressed, and retained such locking-action during depression and until the complete return of such key-depression; thus the keyboard was locked, error or no error. [Sidenote: _Inactive keys not locked during proper key-action in “Comptometer”_] A correct depression of a key in Felt’s new invention, as applied to key-driven calculators, does not lock the rest of the keys. In fact, no key of Felt’s invention is locked until an error occurs. The lock of the key-driven cash register is a lock that takes effect without an error having occurred--one that is always present with respect to the keys not depressed simultaneously, and a feature designed to force simultaneity of group key-action to prevent, as before explained, dishonesty. The lock of the key-driven calculator inventions referred to are in no way connected with simultaneous key-action--as in the cash register--and never act to lock the other orders except when there is an error in a key-stroke. As the writer has explained respecting the simultaneous feature of the cash register, the locking of the other orders in the cash register interfered with the flexibility of the key-action and for that reason would be impossible in a key-driven calculator, where rapid manipulation is dependent on flexibility. The scheme of the new key-driven calculator inventions referred to, were designed to allow perfect freedom of individual key-action and to block such action only when an error in any individual key-stroke should be made. There is nothing in common in the two schemes. The time, purpose and mechanical means employed differ entirely. [Sidenote: “_Controlled-key Comptometer_”] This new idea of Felt’s is embodied in what is commercially known as the “Controlled-key Duplex Comptometer.” The term “Controlled-key” was coined to fit this broadly new combination, but a word coined to fit the functions of a new mechanism is seldom enough to convey a complete understanding of its true qualities. Aside from the broad newness of the Felt “Controlled-key” feature referred to, even the mechanical means for safeguarding the individual key-action was new in its application as a full-stroke device. The means employed operated directly on the accumulator mechanism, locking it against registration until the error was corrected, which differed greatly from the devices applied to the keys or actuators designed by others to bring about a similar result. But the locking of all the other orders of mechanism, through any key-action short of a full stroke, as a signal or error, has no mechanical equivalent or simile in the Art. The Improved Recorder [Sidenote: _The mass of recorder inventions patented_]