The "Reverse Combination" and Why Certain Combinations are Not Used
The standard way of opening the lock is to turn the dial twice to the right and then stop at the first number. Then, turn the dial left one turn and continue to the second number. Finally, right to the last number. Pull the shackle to open the lock. Turning the dial twice to the right ensures that all three cams are turning, so the notch on the back cam will line up under the pawl when the first number is reached. Turning left one turn ensures that the peg on the front cam engages with the peg on the middle cam, and it begins to turn. When the second number is reached, the notch on the middle cam lines up under the pawl. The peg on the back of the middle cam has not engaged with the peg on the back cam, so that cam has not turned, and its notch is still aligned under the pawl. Turning right less than a turn to the third number lines up the notch on the front cam without turning the other two cams.

You may have wondered if the cams could be lined up by starting out with turning the dial two turns to the
LEFT. It turns out that they can. The first number to stop at will be 6 marks before the first number in the "normal" combination. In other words, if the first number were 20, the first number in the reverse combination would be 14. The offset of 6 is due to the width of the pegs. The second number in the reverse combination is 3 marks after the normal second number. Thus, if the second number were 10, the second number in the reverse combination would be 13. The third number is the same.

The offsets 6 and 3 were found experimentally and may be slightly different on some locks.

Note that some of the second numbers were ruled out. For example, if the first number were 0, we did not test 34 or 38 as the second number. If we tried to dial 38, the peg on the middle cam would engage with the peg on the back cam, and the back cam would turn, moving its notch out of alignment. If 34 were the correct second number, it would not be necessary to dial the first number. Just turn the dial twice to the left and then to the second number, and both the middle and back cams would have their notches aligned under the pawl. Turn right to the third number and the lock would open. If each of the possible second numbers were used on an equal number of locks, over 10 percent of the locks would have that deficiency. With millions of locks in use, someone with one of those locks would find out by accident, and the word would get around. Buyers could claim the locks to be defective and return them because they opened with two numbers instead of the advertised three.

The other second numbers that we ruled out are those two marks to either side of the third number. If the second number were two marks before the third mark, the notches on the first and second cams would align when the dial was turned one turn to the right. A two number reverse combination would result. If the second number were two marks after the third number, the lock might open when it was half way between the two numbers when the combination was dialed in the normal way..

The reverse combination is a good way to confuse people who watch you dial the combination. They may see the numbers you stop on, but not notice which way you are turning the dial. When they try those numbers later, dialing in the normal way, the lock won't open.

The reverse combination may be easier to dial. For example, one of my locks has combination 35-25-35. After dialing 35 normally, the dial must be turned nearly two turns left to reach 25, and then nearly a full turn right to reach 35. The reverse combination is 29-28-35. The second number is reached in just slightly over one turn and the last number is about 1/5 turn away from the second number. After turning the dial left twice and stopping at 29, I was able to take hold of the knob and dial the second two numbers without removing my hand.

Click here for a page describing the locks with keyholes on the back.


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Click here for a description of a special technique to find the combination, requiring the use of calipers and other equipment, making use of knowledge in how it works.