Whitney, however, had other sources of profit in his own character and mechanical ability.

As early as 1798 he had turned his talents to the manufacture of firearms.

He had established his shops at Whitneyville, near New Haven; and it was there that he worked out another achievement quite as important economically as the cotton gin, even though the immediate consequences were less spectacular: namely, the principle of standardization or interchangeability in manufacture.
This principle is the very foundation today of all American large-scale production.

The manufacturer produces separately thousands of copies of every part of a complicated machine, confident that an equal number of the complete machine will be assembled and set in motion.

The owner of a motor car, a reaper, a tractor, or a sewing machine, orders, perhaps by telegraph or telephone, a broken or lost part, taking it for granted that the new part can be fitted easily and precisely into the place of the old.
Though it is probable that this idea of standardization, or interchangeability, originated independently in Whitney's mind, and though it is certain that he and one of his neighbors, who will be mentioned presently, were the first manufacturers in the world to carry it out successfully in practice, yet it must be noted that the idea was not entirely new.