...things were custom made. If you wanted a stool, you made a stool. If you wanted 5 stools, you made a stool and a stool and a stool and a stool and a stool. None of the parts of one were directly interchangeable with another.
Then came precision measuring and mechanization, and we got mass production. Now you could make a bin full of stool seats, another bin full of stool legs, and a third bin full of stool leg cross-members. If you needed 5 stools, or 500, you just took them out of the parts bin and assembled them.
Now we're stepping into the realm of mass-customization.