Third enlarged edition, November 2021. This is a technical book aimed at the design and restoration of pianos, fortepianos, harpsichords and clavichords. It contains all the useful theoretical tools and explanations (including new accurate formulas on overspun strings) in order to create a consistent stringing scheme. It is first dedicated to restorers facing complex restoration projects, in particular when the information about original stringing has been lost or altered. It will be of interest as well to professional musicians, organologists, collectors, designers or scientists willing to better understand some of the acoustic design principles of cordophones.