This article examines the productivity of American watch manufacturing technology between 1850 and 1900 as representative of the industries collectively known as the American System of Manufactures. It then compares and contrasts products and responses to market events in British and American horology in the nineteenth century. Finally it weighs factor-substitution models which purport to explain the sharply different British and American responses to mechanization, specifically why the American System of Manufactures is indeed American.