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When once the opinion became established that a lock is rendered secure by virtue of its wards, (a theory which we shall have to discuss in a later page,) much ingenuity was displayed in varying the wards of the lock, the clefts of the key, and the shape of the keyhole. Even if the two former were unchanged, a change in the latter might add to the puzzlement of the arrangement. For instance, in the annexed cut (fig. 8), all the six keys represented may have clefts or cuts exactly alike, all alike adapted to the wards of one particular lock; yet the differences in the thickness of the web are such, that if the keyholes were shaped in conformity therewith, each keyhole would be entered by one of these keys; b and c differing from a in the relative thickness at different points, and d, e, and f having certain curvatures and cavities not to be found in the other three.

fig. 9. Examples to shew the action of “master,” or “skeleton keys.”

But without waiting for the detailed examination of the relative security and insecurity of locks, we may at once shew how simple is the principle which renders the warded system fallacious. In fig. 9 we shall be able to illustrate this. Numbers 1, 2, and 3, all appear very different keys, and it is quite true that neither one would open a lock adapted for either of the other two; and yet the very simple arrangement No. 4 would open all three. This No. 4 is called a skeleton-key; and the relation which it bears to the others may be expressed in the form of a proposition thus: at any point where there is solid metal in all the keys, there must (or may) be solid metal in the corresponding part of the skeleton-key; but at any point where there is a vacancy or cavity in any of the keys, there must be a cavity in the corresponding part of the skeleton-key. If Nos. 1, 2, 3, 4, be examined, this proposition will be found to be borne out; there is so much cavity in No. 4 that it avoids the wards in all the three locks, nothing being required but the tongue of metal to move the bolt. Sometimes, to add to the safety, wards are attached to the front as well as the back plate of the lock; and then there may be a double series of notches required in the key, such as in No. 5; but if this be compared with Nos. 9, 10, 11, it will be found that although no one of the four would open a lock adapted for either of the other three, yet the skeleton-key No. 12 would master them all, having cavities wherever any of the others have cavities. This is the theory of the master-key, by which one key may be made to command many locks. Nos. 6 and 7 have complicated wards; but the key is so much cut up as to be weakened more than is desirable. No. 8 enables us to point out the difference between two distinct classes of keys. Keys with pipes or barrels fitting on a pin or pipe-shaft can only open a lock on one side of the door or box; but a key with a solid stem, as No. 8, has the clefts so cut as to open the lock from either side, as in a street-door lock: it is, in fact, two warded keys fixed end to end, only half of which is employed at one time in opening the lock.