What a key really is
A key is a set of instructions in metal. Each notch on its edge tells one pin inside the lock how high to rise, and the lock opens only when every pin receives the right instruction at the same time. The blade of the key carries those instructions, the shoulder stops the key from going in too far, and the bow, the part you hold, gives you leverage to turn it.
The shape of the blade is also part of the instructions. The grooves running along the length of a key, called the warding, match the keyway of the lock. If the profile is wrong, the key will not even enter, no matter how perfectly the cuts are made. That is the reason a key for one brand will not fit another brand's lock, even when they look similar to the eye.
Because the instructions are physical, they can be copied physically. A duplicate is made by tracing the original and reproducing its cuts on a new blank. That is quick and often reliable, but it depends on how accurate the original is, how accurate the machine is, and how well the blank matches. Each of those can add a small error.
Understanding this is helpful for a practical reason. When a key will not work, it is usually not that the lock is broken. It is that the instructions on the key no longer match what the lock expects, and finding out why is a matter of looking at the key, the blank, and the cuts.
Blanks: the starting piece of metal
Every key starts as a blank, an uncut key with the right profile for a particular keyway. Blanks are made by manufacturers in many shapes, each designed for a family of locks. A key cutter picks a blank that matches the profile of your original key, and the match has to be right in several ways: the shape of the blade, the length, the position of the shoulder, and the style of the bow.
A blank that is close but not exact can cause problems. If the shoulder is slightly off, the key sits a little deeper or shallower in the lock, and the cuts line up with the pins in the wrong place. If the warding is not quite right, the key may drag or refuse to enter. If the metal is softer than the original, it will wear faster and the cuts will drift.
Many blanks look nearly identical, and some are specified by tiny differences. Experienced cutters compare the original against a reference chart and sometimes against several candidate blanks. They also look at the stamped markings on the bow, which often identify the keyway.
Quality varies. Blanks made to tight tolerances produce keys that fit smoothly and last, while poor blanks produce keys that wear quickly or bind. For a key you will use every day, such as a front door key, a good blank is worth the extra care. For a rarely used spare, it matters less, but it still needs to be right.
Cut depths, spacing and the idea of a bitting
The pattern of cuts is called the bitting. Each cut is described by two things: its position along the blade, and its depth. The positions are evenly spaced, with one for each pin in the lock. The depths come from a small set of standard steps, and the manufacturer defines how many steps are allowed for a given keyway.
When a lock is made, each pin position is assigned a depth, and the key must match them. A five-pin lock might use a pattern such as depths in a short sequence of numbers, each from a small range. The lock manufacturer chooses which patterns are valid and can set rules to keep neighboring cuts from being too different, because extreme differences weaken the key.
The depth of each cut also has a tolerance. The key does not need to be perfect, but it needs to be within a small distance of the target for each pin to reach the shear line. If one cut is a bit too shallow, a pin does not rise enough, and the plug will not turn. If one cut is too deep, the pin drops too far, with the same result.
Because the allowed tolerance is so small, even a tiny error matters. A key that is cut with a worn cutter wheel, a misadjusted machine or a slightly bent blank can produce cuts that are off by an amount you cannot see with your eye, but that the lock feels clearly. This is a major reason that copies of keys sometimes feel rough or fail.
How a duplicate is cut from an existing key
The most common method of duplication uses a machine with two parallel positions: one holds the original key, and the other holds the blank. A guide follows the cuts of the original, and a cutting wheel reproduces them on the blank. When the guide moves, the cutter moves with it, and the blank is shaped to match.
The setup matters. The original and the blank must be clamped with their shoulders against the same stop, so that the starting point is identical. If the shoulder of one is not seated properly, the whole pattern is shifted, and the copy will be offset from the original. Careful operators check this before cutting.
After cutting, the blank has sharp edges, called burrs, along the cuts. These must be removed, because burrs can catch inside the lock and cause roughness, or can deposit metal bits in the cylinder. A good cutter brushes or polishes the new key and tests it for smoothness.
Better equipment uses electronic or computerized systems that read the original key and calculate the cuts more precisely. These machines can reduce the error that comes from tracing by hand. Even so, the copy can only be as good as the original it reads, and that leads to the biggest cause of failing copies: a worn original.
Our spare key duplication service is built around checking the original, picking the correct blank, and testing the copy before you leave.
Why copies fail: wear, drift and copies of copies
Keys wear. Every time a key is inserted and turned, metal rubs against metal. Over months and years the cuts round off, the peaks wear down, and the depths change by tiny amounts. A key that has been in use for a long time no longer matches the original pattern, but it may still work because the lock has also worn to fit it.
When you copy a worn key, you copy the wear. The new key has the worn dimensions, but the new key is fresh metal in a lock that has adapted to the old key. The result can be a copy that sticks, needs jiggling, or does not turn at all. People often assume the new copy is defective, when the real issue is that the original had drifted.
Copies of copies make it worse. A duplicate made from a duplicate carries the error of the first duplication plus the error of the second. Each generation adds a small deviation, and after a few generations the key may no longer fit the lock at all. This is called duplication drift.
The fix is to cut from the best source available. If you have an original key from the manufacturer or a key cut by information, use that. If you only have a worn key, a technician may be able to cut a new key by reading the pins in the lock itself, which produces a key matched to the lock as it exists today. In some cases the better solution is to rekey the lock so that a fresh key fits perfectly.
- Copying a worn key copies its wear.
- Copies made from copies add small errors each generation.
- A poor blank can wear fast or bind in the lock.
- Burrs left on the key can cause roughness and debris.
- A worn lock may reject a fresh, accurate key.
Cutting by information instead of by tracing
There is another way to make a key that does not rely on copying an existing one. If the cut depths for a key are known, a technician can cut a key directly from those numbers. This is called cutting by information, and it can produce a precise key without any physical original at all.
Where does the information come from? For some locks, it is stamped or recorded by the manufacturer or the original installer. For automotive keys, it can come from the vehicle's identification information, and for some commercial systems it comes from records kept by the key holder. For many house locks, a technician can determine the cut depths by examining the lock cylinder itself, which reveals the pin lengths.
Cutting by information gives a fresh key that matches the intended pattern, not a worn one. That is why it is the right approach for lost keys, for keys that have been copied many times, and for locks where precision matters. It is also commonly used in car key replacement, where the key may need programming as well. Our pages on car key replacement and lost house key replacement explain how this works in practice.
A responsible locksmith asks for proof of ownership before cutting by information, because the process does not require having the original key in hand. That protects owners from someone who might want a key to a lock that is not theirs.
Restricted, transponder and high-security keys
Not every key is cut on a standard machine. High-security keys have more complex profiles, with cuts on the sides or the face of the blade, or with extra elements such as sliders. They require dedicated cutting equipment and, often, authorization. Restricted keyways are controlled so that blanks are available only to approved sources. Our page on high-security key duplication describes what is involved.
Transponder keys carry a small chip in the head, and the car's immobilizer reads it. The metal part is cut like any other key, but the chip has to be programmed for the key to start the engine. A key that opens the door but does not start the engine has the right cuts and the wrong chip. Programming is a separate step.
Some keys are laser cut, with a groove milled into the side of the blade instead of cuts on the edge. They need a different kind of machine, and the keys are often thicker and sturdier. They are common on newer vehicles and some home locks.
If you have a key that is stamped with a warning not to duplicate, that stamp is not a legal barrier, but it often means the key is part of a controlled system. Do not assume a hardware store cutter can duplicate it. If you are unsure, bring the key to a locksmith and ask.
How to get copies that work the first time
A few habits make a big difference. First, copy from the best key you have. If you have an original that has barely been used, use that. If your only key is worn, consider asking for a fresh key by another method, or rekeying the lock. Second, ask the cutter to test the copy in the lock if possible, not just compare it to the original. Third, check the copy yourself before you leave, by inserting and turning it.
If a copy feels rough, do not force it. Forcing can bend the key or damage the lock. Take it back and ask for adjustment. Light burrs can be cleaned, and small errors may be correctable. If the copy is clearly off, a new cut is better than repeated attempts.
Keep spares for the long run. When you get a new key, make a spare from it soon, and store it safely. Avoid copying a copy more than once. If you manage several keys, label them in a way that does not reveal the address, and keep a record of who has which.
Finally, protect the keys you use daily. Do not use a key as a tool, do not hang a heavy bundle from it in the ignition or the lock, and replace a key that is visibly worn. A worn key damages the lock over time, and that is a harder problem to fix than a new key.
- Copy from the best, least worn key you have.
- Ask for the copy to be tested, and test it yourself before leaving.
- Do not force a copy that feels rough.
- Avoid copying copies of copies.
- Make a spare soon after getting a new key.
- Replace worn keys before they damage the lock.
When to call a mobile locksmith instead of a counter cutter
A counter cutter at a hardware store is fine for common keys when you have a good original. A mobile locksmith is the better choice when something is not straightforward: a lost key, a worn original, a copy that has failed, a transponder or high-security key, a lock that needs rekeying, or a situation where you cannot get to a store.
A mobile technician can also test the copy in your lock right away, which is the real test. If it binds, the technician can adjust it or recut it on the spot. If the lock itself is the problem, the technician can see that and offer to repair, rekey or replace it.
For businesses and rental properties, a mobile locksmith can cut many keys and keep a clean record of the work, and can help plan a key system that is easier to manage over time. Those needs are different from a one-off copy, and the planning matters.
Quotes are given by phone. Tell us about the key, the lock and the situation, and we will explain the options. Complicated jobs may need an on-site visit, and a service call charge is explained on the phone before dispatch. If you have a key that will not work, or a spare you want to be sure about, call and we will help you work out the simplest way to get a key that fits.
Questions people ask
Why does my new copy stick when the original works fine?
Usually because the original is worn and the lock has adjusted to it, while the copy reproduces the worn shape in fresh metal, or because of a small cutting error or a leftover burr. A technician can polish the copy, adjust it, or cut a fresh key by information. Forcing a rough copy can damage the lock, so stop and have it checked.
Is it a problem to copy a copy?
It can be. Each duplication adds a small error, and after a few generations the key may no longer match the lock. It is better to copy from an original or from the best key you have, or to cut by information. If you only have copies, a technician can often produce a fresh key that fits properly.
Can a locksmith make a key without the original?
Often yes. A technician can cut a key by information, or can read the pins inside the lock to determine the cuts. Because this does not require the original, a responsible locksmith will ask for proof that you own or are authorized to access the lock before making the key.
Why will a key that opens the car door not start the engine?
If the key has a transponder chip, the car also needs to recognize the chip. The metal cuts may be right, but without the correct chip programmed to the vehicle, the engine will not run. A technician can program the chip with the right equipment. Tell us the make, model and year when you call.
Can any shop duplicate a key marked Do Not Duplicate?
That stamp is a request and not a physical barrier, but it often means the key belongs to a controlled system with a restricted keyway. A standard hardware counter may not be able to cut it correctly or may not be authorized to. A locksmith can advise on what is possible and what authorization is needed.
How do I get a quote for key cutting?
Quotes are given by phone. Describe the key type, how many copies you want and whether you have an original. Simple jobs can be quoted on the call. If the work is more complicated, an on-site visit may be needed. A service call charge applies and is explained before anyone is dispatched.
Describe the situation by phone and we will tell you what the job involves. Quotes are given by phone. Jobs that cannot be quoted without seeing them need a technician on site, and the service call charge is explained on the phone before anyone is dispatched.
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