Two centuries ago, an industrial revolution changed how we make things. Before the revolution, almost everything was made by hand. After the revolution, most things were made by machines or manufactured on assembly lines.

That industrial revolution was made possible (in part) by interchangeable parts.

Interchangeable parts are specified and manufactured to such close tolerances that you can put them together without having to alter them.

In a few technologies, interchangeable parts have been around for a long time. Movable type was invented over a thousand years ago in China, and was reinvented in Germany about four hundred years later.

During the eighteenth century, however, most things were still built out of crudely handmade parts.

Have you ever heard of a flash in the pan? That's what happens with a flintlock musket when the trigger is pulled, releasing the cock, so its attached flint strikes the frizzen, creating a spark that ignites the gunpowder in the pan --- but the resulting flash *doesn't* pass through the touchhole to ignite the main charge, so the gun doesn't fire.

To make a flintlock musket, a blacksmith had to fabricate the cock, frizzen, pan, spring, and the sideplate that holds all those things together. To make everything fit, he'd file down some of the parts or heat them up and hammer on them, whatever works. If one of those parts were to break, years later, its replacement would have to be custom-fit to the other parts on that particular flintlock mechanism.

In the United States, flintlock muskets and pistols began to be made out of interchangeable parts around 1820.