The viola d'amore usually has six or seven playing strings, which are sounded by drawing a bow across them, just as with a violin. In addition, it has an equal number of sympathetic strings located below the main strings and the fingerboard which are not played directly but vibrate in sympathy with the notes played. A common variation is six playing strings, and instruments exist with as many as fourteen sympathetic strings alone. Despite the fact that the sympathetic strings are now thought of as the most characteristic element of the instrument, early forms of the instrument almost uniformly lacked them. The first unambiguous reference to a viola d'amore without sympathetic strings does not occur until the 1730s. Both types continued to be built and played through the 18th century.