Museo Galileo
Virtual Museum

Unit of measurement for small lengths: in Italian, pollice, from the Latin pollex [thumb]. This usage derives from antiquity, when linear measures referred to parts of the human body. In Greece and Rome, the value of the inch varied from region to region. The harmonization of local measures with those used in Rome, which took place in the reign of Augustus (27 B.C.E - 14 C.E.), led to the general acceptance of the inch as the twelfth part of the foot, the basic unit of the Roman linear system. In the centuries after the break-up of the Roman Empire, the inch was widely adopted for surface and volume measurements as well. Adopted in Europe, the inch remained in use for centuries, with different values from town to town, until the introduction of the decimal metric system (its modern value is c 2.54 cm). The inch continues to be used in the U.S. and other English-speaking countries, where it also serves as a unit of measurement for atmospheric pressure. In Italy, it is used to indicate the diameter of plumbing pipes and the diagonal length of video screens.