The concept of interchangeable parts was used by Christopher Polhem in the manufacture of clock gears in Sweden at the beginning of the 1700s. The gears were made by machines with precision measurement to insure interchangeability; however, this work was probably not known in America. In his book, The Wealth of Nations, published in 1776, Adam Smith had discussed the idea of dividing labor—giving a single task to each worker to perform. By the 1790s, Samuel Bethan and Marc Brunel were using division of labor and machinery in mass-producing wooden pulley blocks for the English Navy. Almost every feature of the American system of manufacturing began in Europe earlier, but industrial progress was hindered by maintaining time-honored methods rather than experimentation.
Interchangeable parts are parts () that are, for practical purposes, identical. They are made to that ensure that they are so nearly identical that they will fit into any assembly of the same type. One such part can freely replace another, without any custom fitting (such as ). This interchangeability allows easy assembly of new devices, and easier repair of existing devices, while minimizing both the time and skill required of the person doing the assembly or repair.
In 1798 Eli Whitney built a firearms factory near New Haven. The muskets his workmen made by methods comparable to those of modern mass industrial production were the first to have standardized, interchangeable parts.
The idea of interchangeable parts and the separate assembly line was not new, though it was little used. The idea was first developed in during the period and later the over 2200 years ago – bronze crossbow triggers and locking mechanisms were mass-produced and made to be interchangeable. during the late Middle Ages had ships that were produced using pre-manufactured parts, , and . The apparently produced nearly one ship every day, in what was effectively the world’s first .