Most people take the concept of interchangeable parts for granted today. By "interchangeable part" is meant a part which is pre-manufactured and stored somewhere. When a corresponding part breaks down, it is replaced by a new, of f the self part. Anybody who has gone to Canadian Tire to buy a replacement fan belt is taking advantage of the interchangeable part system.
It was not always thus. Until the early 19th century everything was hand made by a craftman. Every part was different even when serving the same function. You could not buy a new part. You had to go to a craftman and have it "tailor made" to replace the broken part. Needless to say such a process was time consuming and expensive, and sometimes impossible.
The history of manufacturing in general, and interchangeable parts in particular, is closely tied to military history. European and American armies were always interested in better guns.
In the 18th century guns (pistols and muskets) were all hand made by gunsmiths.
Gun making was a craft. Some pistols were so fancy as to be almost works of art. Pistols and muskets were all essentially one-of-a-kind craft objects. When something broke, repair was difficult. (Sounds a bit like present day software, doesn't it?)
Credit for the idea of interchangeable parts generally goes to the French gunsmith Honore Le Blanc. In the mid 18th century he proposed making gun parts from standardized patterns (an idea that reappears in Java Beans!) Since all the parts would follow the same part, a broken part could be replaced by another, identical part.
LeBlanc didn't get very far with his ideas. Other gunsmiths saw this idea as a threat to their livelihood and opposed it. However, Thomas Jefferson was taken with the idea and brought it to America. Here it gained a more favourable reception and in 1796 the famous American inventor, Eli Whitney was give the contract to make 10,000 muskets using the new methodology.
Eli Whitney 
was one of the most famous American inventors, best known for the . In the long run, however, his most important contribution was this one, the introduction of standardized manufacture with interchangeable parts into American industry. A brief description.
The technique of using interchangeable parts based on standardized design patterns spread throughout American industry. The gun manufactures such as Colt and Smith & Wesson, lead the way. Compare this Colt 45 "Peacemaker" (Colt's name!) 
with t ornheate hand crafted pistols shown above.
With the Peacemaker (and other guns) if Billy the Kid or Wyattt Earp needed a new trigger, they could look for the part in the Sears-Roebuck catalog, and have it shipped in by rail from Chicago.
Other more peaceful manufactures were soon using the technique which became know world-wide as the American System of Manufacture. Singer Sewing Machines, locomotives, bicycles, automobiles. In the 20th century, the assembly line appeared where workers assembled products from standardized parts.
By the end of the 19th century, thanks in part to this "American System of Manufacture", American industry was already out-producing that of every other country, including the then leading powers, Great Britain and Germany.
Some people, knowing this story, view the software production industry in 2000 as being much like the gun industry in 1800! The solution they say is, as before, interchangeable parts. In the software case this means components. In this view Java Beans and Microsoft ActiveX controls point the way to the future. Hand crafted software will disappear.
You be the judge.