Modern locks utilize sets of spring-loaded brass pins to function.

As seen in the diagram, the key raises the bottom pins in the pin stack to the shear line so that the cylinder plug may turn, activating or deactivating the locking mechanism.

These pins come in different lengths, and can be swapped out to change the key. That’s how they mass manufacture locks that aren’t all on the same key. It’s the same line for lock cylinders, but they use different pins and keys in assembly.

That said, there are only so many combinations of depths a key can have. Most residential keys have 5 cuts, and each cut has around 7 possible depths. So, the combinations aren’t infinite but they are varied enough that a large residential area, all using the same type of lock, shouldn’t have any repeats. And if it does, who’s going around testing their key in every door?

A lock only keeps an honest person out. The point of a lock is to make it too much of a hassle for a potential thief to bother breaking in.

Edit: I saw no point in adding a 20th version of how locks are made. I instead opted to tackle a separate misconception in the opening statement.

“Made on the same assembly line means it’s the same product” is a myth from people who have no experience in manufacturing/sourcing and are just mad about inflation and do not have a professional interest in the product. The specs are rarely the same. There are often typically significant differences in material, tooling, QA/QC, and warranty. Yes, there are plenty of examples where the upcharge is not justified, but it’s neither the rule nor the exception. It varies wildly across the market. I have my places where I buy premium, I have my places where I buy bottom tier.

For the common end user of household products, the closest they’ll get to understanding this is buying the Amazon, Alibaba, or Temu “version” of something. There will be a dozen differences that make the product worse. Maybe that’s fine for your use. If you think all toothbrushes are the same, try the free ones from a hotel. The handles are small, weak, and usually have sharp mold parting lines. But sure, they were likely made at the same place that made the $6 Colgate because the bristle-placing machine is the most important part of the process.

Meanwhile, towards the other end, a casual household end user will likely never exceed the capability of a hardware store wrench, so they’ll think it’s insane to pay more for a Snap-on at 4x the price. But it makes a difference to someone using and abusing it 8x a day, depending on its function to get paid. If it does break, the warranty replaces it immediately. Lifetime warranties from non-professional brands are notorious for stating it’s the lifetime of the product, not your lifetime, and it expired when it broke or wore out.

At the extreme end would be something like aircraft parts. The “same” bolt at the local store is 1/20 the price. But the aircraft bolt is a higher grade (more expensive), has much tighter tolerances (more money spent on control, higher scrap rate), has backing traceability documentation (money spent on labor and tracking systems), and is likely checked 100% to dimensional spec (money spent on labor and time). You could find the same bolt at the store. You will find a bolt that’s almost the same. You may find a bolt that’s completely wrong. None of that uncertainty is allowable in an aircraft bolt. Those “minor defects here and there” like your toothbrush claim are not acceptable, so systems must be in place to prevent them from escaping. You order a bolt, you get the bolt you ordered. Hundreds of lives depend on it.

This video explains how standard locks work.

Basically all the parts of the lock are mass produced. Only the pins are unique. And they’re aren’t actually unique, just the combinations of pins are.

When you buy keyed doorknobs and deadbolts, there’s sometimes (always?—not sure, but def sometimes) a sticker on each package with a code. This lets you look through the available inventory to find and buy additional locks with the same code so that if you need multiple locks for the same house, they can all use the same key.

So no, as others have said, mass produced locks aren’t unique, but sometimes that’s a benefit.

Edit to add: it’s okay that locks aren’t unique, because the lock itself isn’t really what keeps people from entering locked doors. Mostly it’s the social contract. Your house key might unlock several houses in your neighborhood, but you’re not gonna try it, because how would you explain yourself if you got caught? And if you weren’t worried about that, then you’d probably be okay with just smashing the window…which means that for someone who would violate it, the lock is moot.

Actually, a lot of locks ARE the same. I lived in a brand-new complex years back, and everyone had the same lock and key. It didn’t take long for some kids to start breaking into other people’s houses, and since it was all under HOA owners couldn’t just buy mismatched locks.

Then when I bought a 10 yr old home later, I was sitting at the house when some maintenance crew unlocked my door and walked right in. They tried to tell me I was squatting and it wasn’t until they realized they were to do work on my neighbors house. Went and checked with other neighbors and found out a handful of us all had the same keys of about 4 varieties. Ironically, there were even two families that knew this before then and they told us how they bought new locks - and found out that those too were the same key.

So basically, if you buy a new lock, you should have it checked by a locksmith and rekeyed if needed.

Edit: There is a lot of side discussion on this post. Feel free to inquire and discuss, but note that context was left out in a lot of places. I understand HOAs can be difficult, but the OP is about locks. Just be mindful that there is more to the story I did not detail.

Each tumbler has 4 to 6 sizes in 3 to 6 holes. They set the combination, then cut the key and put the matching tumblers in a standard housing.

Half the security on most locks is that it’s hard to get tools in the tiny slots. The actual number of combinations is small, but difficult to scan through.

locks arent useful majority can be broken with the right lockpick. Also they mass produce them with variations so that they dont work on another you would find it hilarious how easy one lock can have multiple key configurations by changing the math behind the pins and keys. It’s more to prevent someone doing it easy and just opening the door than an actual burglary. This is one of those inventions that prevents opportunistic behaviour