 The only completely chromatic harps are the double (arpa doppia) and triple (Welsh) harps and cross-strung harp.The pedal mechanism is called the "double-action pedal system", first patented in London by Sebastien Erard in 1801 (patent number 2502) and 1802 (patent number 2595).In 1807 Charles Groll was the first to register a patent (patent number 3059) where the harp mechanism was doubled with two lines of fourchettes (forks).