The operation of key-driven machines has always been attended more or less with a feeling that a key-stroke may not have been completed, especially by a novice in operating. Recognition of the possibility of errors occurring through incomplete key-strokes in key-driven adding mechanism was first disclosed as early as 1872 in the Robjohn patent (see page 36), in which a full-stroke device is shown co-acting with the keys. In the drawings it will be noted that for each key there is provided a ratchet device co-operating with the key to compel a full-stroke. This scheme, like other similar later attempts, was aimed at the prevention of an error in the operation of adding mechanism, but as a means of prevention of an error it was lacking, because unless the operator noticed that the key had not returned the next key depressed would, through the action of the rotor, pull the partly depressed key way down until it was released, when it would rise again, possibly without the knowledge of the operator. There still remained the fact that the occurrence of the error was not made known to the operator until it was too late to correct it. [Sidenote: _Error signal keyboard_] That Felt was interested in the solution of the problem for detection and correction of the errors in key-strokes is shown in the several patents issued to him on features pertaining to this subject. After numerous experiments Felt came to the conclusion that it was futile to lock a key in event of a partial stroke and that the solution lay in the locking of the keys in the other orders from that in which the error had been made, thus signaling the operator and compelling correction before further manipulation could be accomplished. Again we find, as with the simultaneity of key-action, that a question may be raised as to the novelty of invention by those who wish to say that there are full-stroke mechanisms in the key-driven cash register Art that lock the rest of the keyboard. But the key-locks disclosed in the cash register were directed to a continuity of stroke engroup, as distinguished from the individualism necessary to the key-driven calculator. The mechanical means employed, of course, varied greatly from that which would be of any value in the calculating machine Art, and the theoretical scheme was aimed at a widely different result. Flexibility was necessary. [Sidenote: _Locking of the other orders by a short key-stroke_] The feature sought by Felt for his calculator was a signal to the operator that an error had been made--if an error should occur--and to block the operation of any of the other orders until the error was corrected. This he accomplished by causing all the other orders to be locked against manipulation, through the occurrence of an error in a key-stroke; thus preventing manipulation of another order until the error was corrected. [Sidenote: _Inactive keys locked during proper key-action in cash register_]