A modern concert harp has 47 strings but is a fully chromatic instrument thanks to seven pedals at its base. Invented by Sébastien Erard, the 'double-action' pedal system allows each string to be tuned to three different pitches: flat, natural, and sharp. When a pedal is pressed, it engages small discs with 'pins' that shorten the length of all the strings of that note name (e.g., all the C strings). This mechanical complexity allows the harpist to play in any key and perform rapid chromatic passages that were impossible on earlier harps.