The industrial revolution was facilitated by two elements: interchangeability of parts and division of labor. Division of labor has been a feature from the beginning of civilization, the extent to which the division is carried out varied considerably depending on period and location. Compared to the Middle Ages, the Renaissance and the Age of Discovery were characterized by a greater specialization in labor, which was a characteristic of the growing cities and trade networks of Europe. An important leap in manufacturing efficiency came in the late eighteenth century as Eli Whitney popularized the concept of interchangeability of parts when he manufactured 10,000 muskets. Up to this point in the history of manufacturing, each product (e.g. each musket) was considered a special order, meaning that parts of a given musket were fitted only for that particular musket and could not be used in other muskets. Interchangeability of parts allowed the mass production of parts independent of the final products in which they would be used. An entire new market to fill the need for the sale and manufacturing of muskets began at this time.