During the 16th and 17th centuries, ivory diptych sundials were widely used to determine the time by day or night. These elaborate portable sundials, which could be adjusted for different latitudes, incorporated various devices useful for merchants and others who travelled extensively in Europe. This catalogue illustrates in detail Harvard's collection of 83 ivory diptych sundials, one of the largest holdings of these instruments in the world. The collection encompasses a comprehensive array of styles and designs from Nuremberg, Paris, and Dieppe, the major centres of their production, as well as from other parts of Europe. This catalogue is the fourth publication of Harvard University's Collection of Historical Scientific Instruments. This collection, which was established in 1949 as a resource for the history of science and technology, has one of the three largest university holdings of its kind in the world. It comprises about 15,000 instruments covering a broad range of scientific disciplines dating from 1500 to the present.