The Clavichord is a small keyboard instrument popular for domestic use up to the late 18th century. It has undergone a revival in the late 20th century and many instruments are currently built based on historic instruments and practices. The sound is produced when a metal tangent on the back of the key strikes the string. Unlike the piano, which has a complex hammer mechanism, the tangent remains in contact with the string while the note sounds. Clavichords have a soft tone but allow variation in loud and soft unlike a harpsichord.