In the 19th century two young French physicists at the University of Marseilles, Alfred Perot and Charles Fabry, described a novel form of interference device. Underlying much of the use of their interferometer was the phenomenon of 'coincidences'. Fabry and Perot swiftly realized that the sharp, multiple-beam fringes from their equipment offered great precision for size measurement, wavelength comparison and a wide range of applications in metrology and spectroscopy. Fabry and Perot pointed to the sharpness and contrast ratio of fringes arising in a thin film of air between glass plates. In addressing the problem of determining the order of a fringe Fabry and Perot made ingenious use of their method of coincidences. In the measurement of length Fabry and Perot maintained that with super-position fringes it was possible to fix a reference position within a few thousandths of a micrometre. The chapter also presents an overview of the key concepts discussed in this book.