How things work

How a combination lock knows the code

A working model of the three-wheel lock on a safe door, with the back cover off. Every wheel moves only when the dial really would have picked it up.

The dial

Pull apart

Set a different code

What is simplified

The wheels are spaced further apart than in a real lock so you can see the drive pins and flies between them. A real wheel pack is much shallower.
The lock case is drawn with two of its walls cut low, and the cover, the lever spring, the lever stop and the relock trigger are left out.
On a real lock you change the code with a change key through the back, which frees each wheel's toothed inner hub from its outer ring while you dial the new numbers. Here the buttons simply move each gate around its wheel.
The gates here are one mark wider than the fence on each side. The maker of the lock this is modeled on says a good one opens half a mark off, and some open a full mark off.
Makers advise against some codes, for example numbers closer than ten apart, or a last number in the part of the dial where the cam's notch sits under the lever. In this model a last number of 97, 98, 99 or 0 only pulls the bolt part of the way.
As the bolt slides back, the fence is shown nudging the three wheels round with it. That follows from this model's geometry and may differ from a particular real lock.

In short

A safe dial is linked to a stack of wheels, the wheel pack, each with a notch called a gate. Turning the dial in different directions picks up and positions the wheels one by one. When the correct numbers have been entered, every gate lines up. A small bar, the fence, can then drop into the aligned gates, which frees the mechanism that holds the bolt. If any wheel is out of place, the fence is blocked and the bolt stays locked.