My best friend just made a copy of our apartment key. She was pretty sure the machine messed up because the key was cut very different from the original. To her surprise, it still opened our front door. How is this possible?
Assuming it may have something to do with the height or order of how the pins in the lock open/fall?
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A lock that has two pins can still be opened by a key with 5 teeth so long as two of those teeth are correctly spaced and the correct height to move those pins in the right place. The other 3 teeth do not matter. The appearance of the keys can be different, but those two keys are probably the same where it matters if it unlocks the door.
Residential locks are very far from precise pieces of engineering. Close is often close enough. If you have an original unused key around to compare to your daily used key, compare them and you will see what I mean.
While certain tricks exist that allow different keys to open the same lock, it is unlikely that any were used here.
A lock checks the height of a few specific spots on your key. That’s it. If those spots are right, and the key fits in the lock, then nothing else matters.
The spiky teeth on a key are the sections between these spots, left uncut when the key was produced as the cutter only cuts the spots that matter. These spikes can be whatever height, or fully removed, so long as they don’t get in the way.
A common residential lock ’round here has five pins, so it ‘tests’ five spots on the key. You [can see the six ‘test’ spots on this key as the deepest-cut parts of the key, evenly spaced down its length.](https://cdn11.bigcommerce.com/s-euxnf32031/images/stencil/1280×1280/products/900/1601/C123_KEY__18928__98337.1663005833.jpg?c=1?imbypass=on) In this example, three are deep and three are shallower.
Locks are also not always that precise. The age and make of the lock plays a big role here. There is some leeway in the exact height of these spots. Anywhere from a few thousandths of an inch to over twenty thousandths, depending again on the quality and age of the lock.
The lock only checks how tall the key is at certain points. With any given lock usually having between 4 and 10 pins that each check that the key is the right height at just that point. The key lifts each pin so that the pins shear-line is in line with the cylinder, which makes the cylinder able to rotate.
On top of that the system can be masterkeyed. This means that each pin is split into two. Lift it up to a certain height with the regular key and the cylinder is in line with one shear-line. Lift it up to a different height (with the master key) and it’s in line with a different shearline.
Clarifying question: is she comparing the flat spots of the cut? That’s all that really matters. An old key might be smooth travel to each flat spot while a new one might have spikes between each flat spot. Depends on how the cutting machine works. The flat spots are where the pins touch the key when fully inserted.
Look up lock picking videos online to see how locks are operated. An apartment building is most likely using a master key system where the super has one key that works all the door locks in the building. Because who wants to carry a 100 key ring like high school custodians.
What may have happened is thru sheer randomness, wear of the lock (or poor design), your friend’s key may have a combination of master and tenant wafers that work in your lock.
As long as any of the shear lines line up, the tumbler will turn. The key has no way of knowing or caring if the key in the lock is the “master” or “tenant” key.
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